feat(vanilla): add testing infrastructure and tests (NK-wjnczv)

8 files changed, 1447 insertions, 0 deletions

diff --git a/vanilla/node_modules/css-tree/lib/tokenizer/OffsetToLocation.js b/vanilla/node_modules/css-tree/lib/tokenizer/OffsetToLocation.js
new file mode 100644
index 0000000..cc584c0
--- /dev/null
+++ b/vanilla/node_modules/css-tree/lib/tokenizer/OffsetToLocation.js
@@ -0,0 +1,87 @@
+import { adoptBuffer } from './adopt-buffer.js';
+import { isBOM } from './char-code-definitions.js';
+
+const N = 10;
+const F = 12;
+const R = 13;
+
+function computeLinesAndColumns(host) {
+    const source = host.source;
+    const sourceLength = source.length;
+    const startOffset = source.length > 0 ? isBOM(source.charCodeAt(0)) : 0;
+    const lines = adoptBuffer(host.lines, sourceLength);
+    const columns = adoptBuffer(host.columns, sourceLength);
+    let line = host.startLine;
+    let column = host.startColumn;
+
+    for (let i = startOffset; i < sourceLength; i++) {
+        const code = source.charCodeAt(i);
+
+        lines[i] = line;
+        columns[i] = column++;
+
+        if (code === N || code === R || code === F) {
+            if (code === R && i + 1 < sourceLength && source.charCodeAt(i + 1) === N) {
+                i++;
+                lines[i] = line;
+                columns[i] = column;
+            }
+
+            line++;
+            column = 1;
+        }
+    }
+
+    lines[sourceLength] = line;
+    columns[sourceLength] = column;
+
+    host.lines = lines;
+    host.columns = columns;
+    host.computed = true;
+}
+
+export class OffsetToLocation {
+    constructor(source, startOffset, startLine, startColumn) {
+        this.setSource(source, startOffset, startLine, startColumn);
+        this.lines = null;
+        this.columns = null;
+    }
+    setSource(source = '', startOffset = 0, startLine = 1, startColumn = 1) {
+        this.source = source;
+        this.startOffset = startOffset;
+        this.startLine = startLine;
+        this.startColumn = startColumn;
+        this.computed = false;
+    }
+    getLocation(offset, filename) {
+        if (!this.computed) {
+            computeLinesAndColumns(this);
+        }
+
+        return {
+            source: filename,
+            offset: this.startOffset + offset,
+            line: this.lines[offset],
+            column: this.columns[offset]
+        };
+    }
+    getLocationRange(start, end, filename) {
+        if (!this.computed) {
+            computeLinesAndColumns(this);
+        }
+
+        return {
+            source: filename,
+            start: {
+                offset: this.startOffset + start,
+                line: this.lines[start],
+                column: this.columns[start]
+            },
+            end: {
+                offset: this.startOffset + end,
+                line: this.lines[end],
+                column: this.columns[end]
+            }
+        };
+    }
+};
diff --git a/vanilla/node_modules/css-tree/lib/tokenizer/TokenStream.js b/vanilla/node_modules/css-tree/lib/tokenizer/TokenStream.js
new file mode 100644
index 0000000..96d48b7
--- /dev/null
+++ b/vanilla/node_modules/css-tree/lib/tokenizer/TokenStream.js
@@ -0,0 +1,316 @@
+import { adoptBuffer } from './adopt-buffer.js';
+import { cmpStr } from './utils.js';
+import tokenNames from './names.js';
+import {
+    WhiteSpace,
+    Comment,
+    Delim,
+    EOF,
+    Function as FunctionToken,
+    LeftParenthesis,
+    RightParenthesis,
+    LeftSquareBracket,
+    RightSquareBracket,
+    LeftCurlyBracket,
+    RightCurlyBracket
+} from './types.js';
+
+const OFFSET_MASK = 0x00FFFFFF;
+const TYPE_SHIFT = 24;
+const balancePair = new Uint8Array(32); // 32b of memory ought to be enough for anyone (any number of tokens)
+balancePair[FunctionToken] = RightParenthesis;
+balancePair[LeftParenthesis] = RightParenthesis;
+balancePair[LeftSquareBracket] = RightSquareBracket;
+balancePair[LeftCurlyBracket] = RightCurlyBracket;
+
+function isBlockOpenerToken(tokenType) {
+    return balancePair[tokenType] !== 0;
+}
+
+export class TokenStream {
+    constructor(source, tokenize) {
+        this.setSource(source, tokenize);
+    }
+    reset() {
+        this.eof = false;
+        this.tokenIndex = -1;
+        this.tokenType = 0;
+        this.tokenStart = this.firstCharOffset;
+        this.tokenEnd = this.firstCharOffset;
+    }
+    setSource(source = '', tokenize = () => {}) {
+        source = String(source || '');
+
+        const sourceLength = source.length;
+        const offsetAndType = adoptBuffer(this.offsetAndType, source.length + 1); // +1 because of eof-token
+        const balance = adoptBuffer(this.balance, source.length + 1);
+        let tokenCount = 0;
+        let firstCharOffset = -1;
+        let balanceCloseType = 0;
+        let balanceStart = source.length;
+
+        // capture buffers
+        this.offsetAndType = null;
+        this.balance = null;
+        balance.fill(0);
+
+        tokenize(source, (type, start, end) => {
+            const index = tokenCount++;
+
+            // type & offset
+            offsetAndType[index] = (type << TYPE_SHIFT) | end;
+
+            if (firstCharOffset === -1) {
+                firstCharOffset = start;
+            }
+
+            // balance
+            balance[index] = balanceStart;
+
+            if (type === balanceCloseType) {
+                const prevBalanceStart = balance[balanceStart];
+
+                // set reference to balance end for a block opener
+                balance[balanceStart] = index;
+
+                // pop state
+                balanceStart = prevBalanceStart;
+                balanceCloseType = balancePair[offsetAndType[prevBalanceStart] >> TYPE_SHIFT];
+            } else if (isBlockOpenerToken(type)) { // check for FunctionToken, <(-token>, <[-token> and <{-token>
+                // push state
+                balanceStart = index;
+                balanceCloseType = balancePair[type];
+            }
+        });
+
+        // finalize buffers
+        offsetAndType[tokenCount] = (EOF << TYPE_SHIFT) | sourceLength; // <EOF-token>
+        balance[tokenCount] = tokenCount; // prevents false positive balance match with any token
+
+        // reverse references from balance start to end
+        // tokens
+        //   token:   a ( [ b c ] d e ) {
+        //   index:   0 1 2 3 4 5 6 7 8 9
+        // before
+        //   balance: 0 8 5 2 2 2 1 1 1 0
+        //            - > > < < < < < < -
+        // after
+        //   balance: 9 8 5 5 5 2 8 8 1 9
+        //            > > > > > < > > < >
+        for (let i = 0; i < tokenCount; i++) {
+            const balanceStart = balance[i];
+
+            if (balanceStart <= i) {
+                const balanceEnd = balance[balanceStart];
+
+                if (balanceEnd !== i) {
+                    balance[i] = balanceEnd;
+                }
+            } else if (balanceStart > tokenCount) {
+                balance[i] = tokenCount;
+            }
+        }
+
+        // balance[0] = tokenCount;
+
+        this.source = source;
+        this.firstCharOffset = firstCharOffset === -1 ? 0 : firstCharOffset;
+        this.tokenCount = tokenCount;
+        this.offsetAndType = offsetAndType;
+        this.balance = balance;
+
+        this.reset();
+        this.next();
+    }
+
+    lookupType(offset) {
+        offset += this.tokenIndex;
+
+        if (offset < this.tokenCount) {
+            return this.offsetAndType[offset] >> TYPE_SHIFT;
+        }
+
+        return EOF;
+    }
+    lookupTypeNonSC(idx) {
+        for (let offset = this.tokenIndex; offset < this.tokenCount; offset++) {
+            const tokenType = this.offsetAndType[offset] >> TYPE_SHIFT;
+
+            if (tokenType !== WhiteSpace && tokenType !== Comment) {
+                if (idx-- === 0) {
+                    return tokenType;
+                }
+            }
+        }
+
+        return EOF;
+    }
+    lookupOffset(offset) {
+        offset += this.tokenIndex;
+
+        if (offset < this.tokenCount) {
+            return this.offsetAndType[offset - 1] & OFFSET_MASK;
+        }
+
+        return this.source.length;
+    }
+    lookupOffsetNonSC(idx) {
+        for (let offset = this.tokenIndex; offset < this.tokenCount; offset++) {
+            const tokenType = this.offsetAndType[offset] >> TYPE_SHIFT;
+
+            if (tokenType !== WhiteSpace && tokenType !== Comment) {
+                if (idx-- === 0) {
+                    return offset - this.tokenIndex;
+                }
+            }
+        }
+
+        return EOF;
+    }
+    lookupValue(offset, referenceStr) {
+        offset += this.tokenIndex;
+
+        if (offset < this.tokenCount) {
+            return cmpStr(
+                this.source,
+                this.offsetAndType[offset - 1] & OFFSET_MASK,
+                this.offsetAndType[offset] & OFFSET_MASK,
+                referenceStr
+            );
+        }
+
+        return false;
+    }
+    getTokenStart(tokenIndex) {
+        if (tokenIndex === this.tokenIndex) {
+            return this.tokenStart;
+        }
+
+        if (tokenIndex > 0) {
+            return tokenIndex < this.tokenCount
+                ? this.offsetAndType[tokenIndex - 1] & OFFSET_MASK
+                : this.offsetAndType[this.tokenCount] & OFFSET_MASK;
+        }
+
+        return this.firstCharOffset;
+    }
+    substrToCursor(start) {
+        return this.source.substring(start, this.tokenStart);
+    }
+
+    isBalanceEdge(pos) {
+        return this.balance[this.tokenIndex] < pos;
+        // return this.balance[this.balance[pos]] !== this.tokenIndex;
+    }
+    isDelim(code, offset) {
+        if (offset) {
+            return (
+                this.lookupType(offset) === Delim &&
+                this.source.charCodeAt(this.lookupOffset(offset)) === code
+            );
+        }
+
+        return (
+            this.tokenType === Delim &&
+            this.source.charCodeAt(this.tokenStart) === code
+        );
+    }
+
+    skip(tokenCount) {
+        let next = this.tokenIndex + tokenCount;
+
+        if (next < this.tokenCount) {
+            this.tokenIndex = next;
+            this.tokenStart = this.offsetAndType[next - 1] & OFFSET_MASK;
+            next = this.offsetAndType[next];
+            this.tokenType = next >> TYPE_SHIFT;
+            this.tokenEnd = next & OFFSET_MASK;
+        } else {
+            this.tokenIndex = this.tokenCount;
+            this.next();
+        }
+    }
+    next() {
+        let next = this.tokenIndex + 1;
+
+        if (next < this.tokenCount) {
+            this.tokenIndex = next;
+            this.tokenStart = this.tokenEnd;
+            next = this.offsetAndType[next];
+            this.tokenType = next >> TYPE_SHIFT;
+            this.tokenEnd = next & OFFSET_MASK;
+        } else {
+            this.eof = true;
+            this.tokenIndex = this.tokenCount;
+            this.tokenType = EOF;
+            this.tokenStart = this.tokenEnd = this.source.length;
+        }
+    }
+    skipSC() {
+        while (this.tokenType === WhiteSpace || this.tokenType === Comment) {
+            this.next();
+        }
+    }
+    skipUntilBalanced(startToken, stopConsume) {
+        let cursor = startToken;
+        let balanceEnd = 0;
+        let offset = 0;
+
+        loop:
+        for (; cursor < this.tokenCount; cursor++) {
+            balanceEnd = this.balance[cursor];
+
+            // stop scanning on balance edge that points to offset before start token
+            if (balanceEnd < startToken) {
+                break loop;
+            }
+
+            offset = cursor > 0 ? this.offsetAndType[cursor - 1] & OFFSET_MASK : this.firstCharOffset;
+
+            // check stop condition
+            switch (stopConsume(this.source.charCodeAt(offset))) {
+                case 1: // just stop
+                    break loop;
+
+                case 2: // stop & included
+                    cursor++;
+                    break loop;
+
+                default:
+                    // fast forward to the end of balanced block for an open block tokens
+                    if (isBlockOpenerToken(this.offsetAndType[cursor] >> TYPE_SHIFT)) {
+                        cursor = balanceEnd;
+                    }
+            }
+        }
+
+        this.skip(cursor - this.tokenIndex);
+    }
+
+    forEachToken(fn) {
+        for (let i = 0, offset = this.firstCharOffset; i < this.tokenCount; i++) {
+            const start = offset;
+            const item = this.offsetAndType[i];
+            const end = item & OFFSET_MASK;
+            const type = item >> TYPE_SHIFT;
+
+            offset = end;
+
+            fn(type, start, end, i);
+        }
+    }
+    dump() {
+        const tokens = new Array(this.tokenCount);
+
+        this.forEachToken((type, start, end, index) => {
+            tokens[index] = {
+                idx: index,
+                type: tokenNames[type],
+                chunk: this.source.substring(start, end),
+                balance: this.balance[index]
+            };
+        });
+
+        return tokens;
+    }
+};
diff --git a/vanilla/node_modules/css-tree/lib/tokenizer/adopt-buffer.js b/vanilla/node_modules/css-tree/lib/tokenizer/adopt-buffer.js
new file mode 100644
index 0000000..ab4566d
--- /dev/null
+++ b/vanilla/node_modules/css-tree/lib/tokenizer/adopt-buffer.js
@@ -0,0 +1,9 @@
+const MIN_SIZE = 16 * 1024;
+
+export function adoptBuffer(buffer = null, size) {
+    if (buffer === null || buffer.length < size) {
+        return new Uint32Array(Math.max(size + 1024, MIN_SIZE));
+    }
+
+    return buffer;
+};
diff --git a/vanilla/node_modules/css-tree/lib/tokenizer/char-code-definitions.js b/vanilla/node_modules/css-tree/lib/tokenizer/char-code-definitions.js
new file mode 100644
index 0000000..715572a
--- /dev/null
+++ b/vanilla/node_modules/css-tree/lib/tokenizer/char-code-definitions.js
@@ -0,0 +1,212 @@
+const EOF = 0;
+
+// https://drafts.csswg.org/css-syntax-3/
+// § 4.2. Definitions
+
+// digit
+// A code point between U+0030 DIGIT ZERO (0) and U+0039 DIGIT NINE (9).
+export function isDigit(code) {
+    return code >= 0x0030 && code <= 0x0039;
+}
+
+// hex digit
+// A digit, or a code point between U+0041 LATIN CAPITAL LETTER A (A) and U+0046 LATIN CAPITAL LETTER F (F),
+// or a code point between U+0061 LATIN SMALL LETTER A (a) and U+0066 LATIN SMALL LETTER F (f).
+export function isHexDigit(code) {
+    return (
+        isDigit(code) || // 0 .. 9
+        (code >= 0x0041 && code <= 0x0046) || // A .. F
+        (code >= 0x0061 && code <= 0x0066)    // a .. f
+    );
+}
+
+// uppercase letter
+// A code point between U+0041 LATIN CAPITAL LETTER A (A) and U+005A LATIN CAPITAL LETTER Z (Z).
+export function isUppercaseLetter(code) {
+    return code >= 0x0041 && code <= 0x005A;
+}
+
+// lowercase letter
+// A code point between U+0061 LATIN SMALL LETTER A (a) and U+007A LATIN SMALL LETTER Z (z).
+export function isLowercaseLetter(code) {
+    return code >= 0x0061 && code <= 0x007A;
+}
+
+// letter
+// An uppercase letter or a lowercase letter.
+export function isLetter(code) {
+    return isUppercaseLetter(code) || isLowercaseLetter(code);
+}
+
+// non-ASCII code point
+// A code point with a value equal to or greater than U+0080 <control>.
+//
+// 2024-09-02: The latest spec narrows the range for non-ASCII characters (see https://github.com/csstree/csstree/issues/188).
+// However, all modern browsers support a wider range, and strictly following the latest spec could result
+// in some CSS being parsed incorrectly, even though it works in the browser. Therefore, this function adheres
+// to the previous, broader definition of non-ASCII characters.
+export function isNonAscii(code) {
+    return code >= 0x0080;
+}
+
+// name-start code point
+// A letter, a non-ASCII code point, or U+005F LOW LINE (_).
+export function isNameStart(code) {
+    return isLetter(code) || isNonAscii(code) || code === 0x005F;
+}
+
+// name code point
+// A name-start code point, a digit, or U+002D HYPHEN-MINUS (-).
+export function isName(code) {
+    return isNameStart(code) || isDigit(code) || code === 0x002D;
+}
+
+// non-printable code point
+// A code point between U+0000 NULL and U+0008 BACKSPACE, or U+000B LINE TABULATION,
+// or a code point between U+000E SHIFT OUT and U+001F INFORMATION SEPARATOR ONE, or U+007F DELETE.
+export function isNonPrintable(code) {
+    return (
+        (code >= 0x0000 && code <= 0x0008) ||
+        (code === 0x000B) ||
+        (code >= 0x000E && code <= 0x001F) ||
+        (code === 0x007F)
+    );
+}
+
+// newline
+// U+000A LINE FEED. Note that U+000D CARRIAGE RETURN and U+000C FORM FEED are not included in this definition,
+// as they are converted to U+000A LINE FEED during preprocessing.
+// TODO: we doesn't do a preprocessing, so check a code point for U+000D CARRIAGE RETURN and U+000C FORM FEED
+export function isNewline(code) {
+    return code === 0x000A || code === 0x000D || code === 0x000C;
+}
+
+// whitespace
+// A newline, U+0009 CHARACTER TABULATION, or U+0020 SPACE.
+export function isWhiteSpace(code) {
+    return isNewline(code) || code === 0x0020 || code === 0x0009;
+}
+
+// § 4.3.8. Check if two code points are a valid escape
+export function isValidEscape(first, second) {
+    // If the first code point is not U+005C REVERSE SOLIDUS (\), return false.
+    if (first !== 0x005C) {
+        return false;
+    }
+
+    // Otherwise, if the second code point is a newline or EOF, return false.
+    if (isNewline(second) || second === EOF) {
+        return false;
+    }
+
+    // Otherwise, return true.
+    return true;
+}
+
+// § 4.3.9. Check if three code points would start an identifier
+export function isIdentifierStart(first, second, third) {
+    // Look at the first code point:
+
+    // U+002D HYPHEN-MINUS
+    if (first === 0x002D) {
+        // If the second code point is a name-start code point or a U+002D HYPHEN-MINUS,
+        // or the second and third code points are a valid escape, return true. Otherwise, return false.
+        return (
+            isNameStart(second) ||
+            second === 0x002D ||
+            isValidEscape(second, third)
+        );
+    }
+
+    // name-start code point
+    if (isNameStart(first)) {
+        // Return true.
+        return true;
+    }
+
+    // U+005C REVERSE SOLIDUS (\)
+    if (first === 0x005C) {
+        // If the first and second code points are a valid escape, return true. Otherwise, return false.
+        return isValidEscape(first, second);
+    }
+
+    // anything else
+    // Return false.
+    return false;
+}
+
+// § 4.3.10. Check if three code points would start a number
+export function isNumberStart(first, second, third) {
+    // Look at the first code point:
+
+    // U+002B PLUS SIGN (+)
+    // U+002D HYPHEN-MINUS (-)
+    if (first === 0x002B || first === 0x002D) {
+        // If the second code point is a digit, return true.
+        if (isDigit(second)) {
+            return 2;
+        }
+
+        // Otherwise, if the second code point is a U+002E FULL STOP (.)
+        // and the third code point is a digit, return true.
+        // Otherwise, return false.
+        return second === 0x002E && isDigit(third) ? 3 : 0;
+    }
+
+    // U+002E FULL STOP (.)
+    if (first === 0x002E) {
+        // If the second code point is a digit, return true. Otherwise, return false.
+        return isDigit(second) ? 2 : 0;
+    }
+
+    // digit
+    if (isDigit(first)) {
+        // Return true.
+        return 1;
+    }
+
+    // anything else
+    // Return false.
+    return 0;
+}
+
+//
+// Misc
+//
+
+// detect BOM (https://en.wikipedia.org/wiki/Byte_order_mark)
+export function isBOM(code) {
+    // UTF-16BE
+    if (code === 0xFEFF) {
+        return 1;
+    }
+
+    // UTF-16LE
+    if (code === 0xFFFE) {
+        return 1;
+    }
+
+    return 0;
+}
+
+// Fast code category
+// Only ASCII code points has a special meaning, that's why we define a maps for 0..127 codes only
+const CATEGORY = new Array(0x80);
+export const EofCategory = 0x80;
+export const WhiteSpaceCategory = 0x82;
+export const DigitCategory = 0x83;
+export const NameStartCategory = 0x84;
+export const NonPrintableCategory = 0x85;
+
+for (let i = 0; i < CATEGORY.length; i++) {
+    CATEGORY[i] =
+        isWhiteSpace(i) && WhiteSpaceCategory ||
+        isDigit(i) && DigitCategory ||
+        isNameStart(i) && NameStartCategory ||
+        isNonPrintable(i) && NonPrintableCategory ||
+        i || EofCategory;
+}
+
+export function charCodeCategory(code) {
+    return code < 0x80 ? CATEGORY[code] : NameStartCategory;
+}
diff --git a/vanilla/node_modules/css-tree/lib/tokenizer/index.js b/vanilla/node_modules/css-tree/lib/tokenizer/index.js
new file mode 100644
index 0000000..16df44c
--- /dev/null
+++ b/vanilla/node_modules/css-tree/lib/tokenizer/index.js
@@ -0,0 +1,513 @@
+import * as TYPE from './types.js';
+import {
+    isNewline,
+    isName,
+    isValidEscape,
+    isNumberStart,
+    isIdentifierStart,
+    isBOM,
+    charCodeCategory,
+    WhiteSpaceCategory,
+    DigitCategory,
+    NameStartCategory,
+    NonPrintableCategory
+} from './char-code-definitions.js';
+import {
+    cmpStr,
+    getNewlineLength,
+    findWhiteSpaceEnd,
+    consumeEscaped,
+    consumeName,
+    consumeNumber,
+    consumeBadUrlRemnants
+} from './utils.js';
+
+export function tokenize(source, onToken) {
+    function getCharCode(offset) {
+        return offset < sourceLength ? source.charCodeAt(offset) : 0;
+    }
+
+    // § 4.3.3. Consume a numeric token
+    function consumeNumericToken() {
+        // Consume a number and let number be the result.
+        offset = consumeNumber(source, offset);
+
+        // If the next 3 input code points would start an identifier, then:
+        if (isIdentifierStart(getCharCode(offset), getCharCode(offset + 1), getCharCode(offset + 2))) {
+            // Create a <dimension-token> with the same value and type flag as number, and a unit set initially to the empty string.
+            // Consume a name. Set the <dimension-token>’s unit to the returned value.
+            // Return the <dimension-token>.
+            type = TYPE.Dimension;
+            offset = consumeName(source, offset);
+            return;
+        }
+
+        // Otherwise, if the next input code point is U+0025 PERCENTAGE SIGN (%), consume it.
+        if (getCharCode(offset) === 0x0025) {
+            // Create a <percentage-token> with the same value as number, and return it.
+            type = TYPE.Percentage;
+            offset++;
+            return;
+        }
+
+        // Otherwise, create a <number-token> with the same value and type flag as number, and return it.
+        type = TYPE.Number;
+    }
+
+    // § 4.3.4. Consume an ident-like token
+    function consumeIdentLikeToken() {
+        const nameStartOffset = offset;
+
+        // Consume a name, and let string be the result.
+        offset = consumeName(source, offset);
+
+        // If string’s value is an ASCII case-insensitive match for "url",
+        // and the next input code point is U+0028 LEFT PARENTHESIS ((), consume it.
+        if (cmpStr(source, nameStartOffset, offset, 'url') && getCharCode(offset) === 0x0028) {
+            // While the next two input code points are whitespace, consume the next input code point.
+            offset = findWhiteSpaceEnd(source, offset + 1);
+
+            // If the next one or two input code points are U+0022 QUOTATION MARK ("), U+0027 APOSTROPHE ('),
+            // or whitespace followed by U+0022 QUOTATION MARK (") or U+0027 APOSTROPHE ('),
+            // then create a <function-token> with its value set to string and return it.
+            if (getCharCode(offset) === 0x0022 ||
+                getCharCode(offset) === 0x0027) {
+                type = TYPE.Function;
+                offset = nameStartOffset + 4;
+                return;
+            }
+
+            // Otherwise, consume a url token, and return it.
+            consumeUrlToken();
+            return;
+        }
+
+        // Otherwise, if the next input code point is U+0028 LEFT PARENTHESIS ((), consume it.
+        // Create a <function-token> with its value set to string and return it.
+        if (getCharCode(offset) === 0x0028) {
+            type = TYPE.Function;
+            offset++;
+            return;
+        }
+
+        // Otherwise, create an <ident-token> with its value set to string and return it.
+        type = TYPE.Ident;
+    }
+
+    // § 4.3.5. Consume a string token
+    function consumeStringToken(endingCodePoint) {
+        // This algorithm may be called with an ending code point, which denotes the code point
+        // that ends the string. If an ending code point is not specified,
+        // the current input code point is used.
+        if (!endingCodePoint) {
+            endingCodePoint = getCharCode(offset++);
+        }
+
+        // Initially create a <string-token> with its value set to the empty string.
+        type = TYPE.String;
+
+        // Repeatedly consume the next input code point from the stream:
+        for (; offset < source.length; offset++) {
+            const code = source.charCodeAt(offset);
+
+            switch (charCodeCategory(code)) {
+                // ending code point
+                case endingCodePoint:
+                    // Return the <string-token>.
+                    offset++;
+                    return;
+
+                    // EOF
+                    // case EofCategory:
+                    // This is a parse error. Return the <string-token>.
+                    // return;
+
+                // newline
+                case WhiteSpaceCategory:
+                    if (isNewline(code)) {
+                        // This is a parse error. Reconsume the current input code point,
+                        // create a <bad-string-token>, and return it.
+                        offset += getNewlineLength(source, offset, code);
+                        type = TYPE.BadString;
+                        return;
+                    }
+                    break;
+
+                // U+005C REVERSE SOLIDUS (\)
+                case 0x005C:
+                    // If the next input code point is EOF, do nothing.
+                    if (offset === source.length - 1) {
+                        break;
+                    }
+
+                    const nextCode = getCharCode(offset + 1);
+
+                    // Otherwise, if the next input code point is a newline, consume it.
+                    if (isNewline(nextCode)) {
+                        offset += getNewlineLength(source, offset + 1, nextCode);
+                    } else if (isValidEscape(code, nextCode)) {
+                        // Otherwise, (the stream starts with a valid escape) consume
+                        // an escaped code point and append the returned code point to
+                        // the <string-token>’s value.
+                        offset = consumeEscaped(source, offset) - 1;
+                    }
+                    break;
+
+                // anything else
+                // Append the current input code point to the <string-token>’s value.
+            }
+        }
+    }
+
+    // § 4.3.6. Consume a url token
+    // Note: This algorithm assumes that the initial "url(" has already been consumed.
+    // This algorithm also assumes that it’s being called to consume an "unquoted" value, like url(foo).
+    // A quoted value, like url("foo"), is parsed as a <function-token>. Consume an ident-like token
+    // automatically handles this distinction; this algorithm shouldn’t be called directly otherwise.
+    function consumeUrlToken() {
+        // Initially create a <url-token> with its value set to the empty string.
+        type = TYPE.Url;
+
+        // Consume as much whitespace as possible.
+        offset = findWhiteSpaceEnd(source, offset);
+
+        // Repeatedly consume the next input code point from the stream:
+        for (; offset < source.length; offset++) {
+            const code = source.charCodeAt(offset);
+
+            switch (charCodeCategory(code)) {
+                // U+0029 RIGHT PARENTHESIS ())
+                case 0x0029:
+                    // Return the <url-token>.
+                    offset++;
+                    return;
+
+                    // EOF
+                    // case EofCategory:
+                    // This is a parse error. Return the <url-token>.
+                    // return;
+
+                // whitespace
+                case WhiteSpaceCategory:
+                    // Consume as much whitespace as possible.
+                    offset = findWhiteSpaceEnd(source, offset);
+
+                    // If the next input code point is U+0029 RIGHT PARENTHESIS ()) or EOF,
+                    // consume it and return the <url-token>
+                    // (if EOF was encountered, this is a parse error);
+                    if (getCharCode(offset) === 0x0029 || offset >= source.length) {
+                        if (offset < source.length) {
+                            offset++;
+                        }
+                        return;
+                    }
+
+                    // otherwise, consume the remnants of a bad url, create a <bad-url-token>,
+                    // and return it.
+                    offset = consumeBadUrlRemnants(source, offset);
+                    type = TYPE.BadUrl;
+                    return;
+
+                // U+0022 QUOTATION MARK (")
+                // U+0027 APOSTROPHE (')
+                // U+0028 LEFT PARENTHESIS (()
+                // non-printable code point
+                case 0x0022:
+                case 0x0027:
+                case 0x0028:
+                case NonPrintableCategory:
+                    // This is a parse error. Consume the remnants of a bad url,
+                    // create a <bad-url-token>, and return it.
+                    offset = consumeBadUrlRemnants(source, offset);
+                    type = TYPE.BadUrl;
+                    return;
+
+                // U+005C REVERSE SOLIDUS (\)
+                case 0x005C:
+                    // If the stream starts with a valid escape, consume an escaped code point and
+                    // append the returned code point to the <url-token>’s value.
+                    if (isValidEscape(code, getCharCode(offset + 1))) {
+                        offset = consumeEscaped(source, offset) - 1;
+                        break;
+                    }
+
+                    // Otherwise, this is a parse error. Consume the remnants of a bad url,
+                    // create a <bad-url-token>, and return it.
+                    offset = consumeBadUrlRemnants(source, offset);
+                    type = TYPE.BadUrl;
+                    return;
+
+                // anything else
+                // Append the current input code point to the <url-token>’s value.
+            }
+        }
+    }
+
+    // ensure source is a string
+    source = String(source || '');
+
+    const sourceLength = source.length;
+    let start = isBOM(getCharCode(0));
+    let offset = start;
+    let type;
+
+    // https://drafts.csswg.org/css-syntax-3/#consume-token
+    // § 4.3.1. Consume a token
+    while (offset < sourceLength) {
+        const code = source.charCodeAt(offset);
+
+        switch (charCodeCategory(code)) {
+            // whitespace
+            case WhiteSpaceCategory:
+                // Consume as much whitespace as possible. Return a <whitespace-token>.
+                type = TYPE.WhiteSpace;
+                offset = findWhiteSpaceEnd(source, offset + 1);
+                break;
+
+            // U+0022 QUOTATION MARK (")
+            case 0x0022:
+                // Consume a string token and return it.
+                consumeStringToken();
+                break;
+
+            // U+0023 NUMBER SIGN (#)
+            case 0x0023:
+                // If the next input code point is a name code point or the next two input code points are a valid escape, then:
+                if (isName(getCharCode(offset + 1)) || isValidEscape(getCharCode(offset + 1), getCharCode(offset + 2))) {
+                    // Create a <hash-token>.
+                    type = TYPE.Hash;
+
+                    // If the next 3 input code points would start an identifier, set the <hash-token>’s type flag to "id".
+                    // if (isIdentifierStart(getCharCode(offset + 1), getCharCode(offset + 2), getCharCode(offset + 3))) {
+                    //     // TODO: set id flag
+                    // }
+
+                    // Consume a name, and set the <hash-token>’s value to the returned string.
+                    offset = consumeName(source, offset + 1);
+
+                    // Return the <hash-token>.
+                } else {
+                    // Otherwise, return a <delim-token> with its value set to the current input code point.
+                    type = TYPE.Delim;
+                    offset++;
+                }
+
+                break;
+
+            // U+0027 APOSTROPHE (')
+            case 0x0027:
+                // Consume a string token and return it.
+                consumeStringToken();
+                break;
+
+            // U+0028 LEFT PARENTHESIS (()
+            case 0x0028:
+                // Return a <(-token>.
+                type = TYPE.LeftParenthesis;
+                offset++;
+                break;
+
+            // U+0029 RIGHT PARENTHESIS ())
+            case 0x0029:
+                // Return a <)-token>.
+                type = TYPE.RightParenthesis;
+                offset++;
+                break;
+
+            // U+002B PLUS SIGN (+)
+            case 0x002B:
+                // If the input stream starts with a number, ...
+                if (isNumberStart(code, getCharCode(offset + 1), getCharCode(offset + 2))) {
+                    // ... reconsume the current input code point, consume a numeric token, and return it.
+                    consumeNumericToken();
+                } else {
+                    // Otherwise, return a <delim-token> with its value set to the current input code point.
+                    type = TYPE.Delim;
+                    offset++;
+                }
+                break;
+
+            // U+002C COMMA (,)
+            case 0x002C:
+                // Return a <comma-token>.
+                type = TYPE.Comma;
+                offset++;
+                break;
+
+            // U+002D HYPHEN-MINUS (-)
+            case 0x002D:
+                // If the input stream starts with a number, reconsume the current input code point, consume a numeric token, and return it.
+                if (isNumberStart(code, getCharCode(offset + 1), getCharCode(offset + 2))) {
+                    consumeNumericToken();
+                } else {
+                    // Otherwise, if the next 2 input code points are U+002D HYPHEN-MINUS U+003E GREATER-THAN SIGN (->), consume them and return a <CDC-token>.
+                    if (getCharCode(offset + 1) === 0x002D &&
+                        getCharCode(offset + 2) === 0x003E) {
+                        type = TYPE.CDC;
+                        offset = offset + 3;
+                    } else {
+                        // Otherwise, if the input stream starts with an identifier, ...
+                        if (isIdentifierStart(code, getCharCode(offset + 1), getCharCode(offset + 2))) {
+                            // ... reconsume the current input code point, consume an ident-like token, and return it.
+                            consumeIdentLikeToken();
+                        } else {
+                            // Otherwise, return a <delim-token> with its value set to the current input code point.
+                            type = TYPE.Delim;
+                            offset++;
+                        }
+                    }
+                }
+                break;
+
+            // U+002E FULL STOP (.)
+            case 0x002E:
+                // If the input stream starts with a number, ...
+                if (isNumberStart(code, getCharCode(offset + 1), getCharCode(offset + 2))) {
+                    // ... reconsume the current input code point, consume a numeric token, and return it.
+                    consumeNumericToken();
+                } else {
+                    // Otherwise, return a <delim-token> with its value set to the current input code point.
+                    type = TYPE.Delim;
+                    offset++;
+                }
+
+                break;
+
+            // U+002F SOLIDUS (/)
+            case 0x002F:
+                // If the next two input code point are U+002F SOLIDUS (/) followed by a U+002A ASTERISK (*),
+                if (getCharCode(offset + 1) === 0x002A) {
+                    // ... consume them and all following code points up to and including the first U+002A ASTERISK (*)
+                    // followed by a U+002F SOLIDUS (/), or up to an EOF code point.
+                    type = TYPE.Comment;
+                    offset = source.indexOf('*/', offset + 2);
+                    offset = offset === -1 ? source.length : offset + 2;
+                } else {
+                    type = TYPE.Delim;
+                    offset++;
+                }
+                break;
+
+            // U+003A COLON (:)
+            case 0x003A:
+                // Return a <colon-token>.
+                type = TYPE.Colon;
+                offset++;
+                break;
+
+            // U+003B SEMICOLON (;)
+            case 0x003B:
+                // Return a <semicolon-token>.
+                type = TYPE.Semicolon;
+                offset++;
+                break;
+
+            // U+003C LESS-THAN SIGN (<)
+            case 0x003C:
+                // If the next 3 input code points are U+0021 EXCLAMATION MARK U+002D HYPHEN-MINUS U+002D HYPHEN-MINUS (!--), ...
+                if (getCharCode(offset + 1) === 0x0021 &&
+                    getCharCode(offset + 2) === 0x002D &&
+                    getCharCode(offset + 3) === 0x002D) {
+                    // ... consume them and return a <CDO-token>.
+                    type = TYPE.CDO;
+                    offset = offset + 4;
+                } else {
+                    // Otherwise, return a <delim-token> with its value set to the current input code point.
+                    type = TYPE.Delim;
+                    offset++;
+                }
+
+                break;
+
+            // U+0040 COMMERCIAL AT (@)
+            case 0x0040:
+                // If the next 3 input code points would start an identifier, ...
+                if (isIdentifierStart(getCharCode(offset + 1), getCharCode(offset + 2), getCharCode(offset + 3))) {
+                    // ... consume a name, create an <at-keyword-token> with its value set to the returned value, and return it.
+                    type = TYPE.AtKeyword;
+                    offset = consumeName(source, offset + 1);
+                } else {
+                    // Otherwise, return a <delim-token> with its value set to the current input code point.
+                    type = TYPE.Delim;
+                    offset++;
+                }
+
+                break;
+
+            // U+005B LEFT SQUARE BRACKET ([)
+            case 0x005B:
+                // Return a <[-token>.
+                type = TYPE.LeftSquareBracket;
+                offset++;
+                break;
+
+            // U+005C REVERSE SOLIDUS (\)
+            case 0x005C:
+                // If the input stream starts with a valid escape, ...
+                if (isValidEscape(code, getCharCode(offset + 1))) {
+                    // ... reconsume the current input code point, consume an ident-like token, and return it.
+                    consumeIdentLikeToken();
+                } else {
+                    // Otherwise, this is a parse error. Return a <delim-token> with its value set to the current input code point.
+                    type = TYPE.Delim;
+                    offset++;
+                }
+                break;
+
+            // U+005D RIGHT SQUARE BRACKET (])
+            case 0x005D:
+                // Return a <]-token>.
+                type = TYPE.RightSquareBracket;
+                offset++;
+                break;
+
+            // U+007B LEFT CURLY BRACKET ({)
+            case 0x007B:
+                // Return a <{-token>.
+                type = TYPE.LeftCurlyBracket;
+                offset++;
+                break;
+
+            // U+007D RIGHT CURLY BRACKET (})
+            case 0x007D:
+                // Return a <}-token>.
+                type = TYPE.RightCurlyBracket;
+                offset++;
+                break;
+
+            // digit
+            case DigitCategory:
+                // Reconsume the current input code point, consume a numeric token, and return it.
+                consumeNumericToken();
+                break;
+
+            // name-start code point
+            case NameStartCategory:
+                // Reconsume the current input code point, consume an ident-like token, and return it.
+                consumeIdentLikeToken();
+                break;
+
+                // EOF
+                // case EofCategory:
+                // Return an <EOF-token>.
+                // break;
+
+            // anything else
+            default:
+                // Return a <delim-token> with its value set to the current input code point.
+                type = TYPE.Delim;
+                offset++;
+        }
+
+        // put token to stream
+        onToken(type, start, start = offset);
+    }
+}
+
+export * from './types.js';
+export * as tokenTypes from './types.js';
+export { default as tokenNames } from './names.js';
+export * from './char-code-definitions.js';
+export * from './utils.js';
+export * from './OffsetToLocation.js';
+export * from './TokenStream.js';
diff --git a/vanilla/node_modules/css-tree/lib/tokenizer/names.js b/vanilla/node_modules/css-tree/lib/tokenizer/names.js
new file mode 100644
index 0000000..54831bd
--- /dev/null
+++ b/vanilla/node_modules/css-tree/lib/tokenizer/names.js
@@ -0,0 +1,28 @@
+export default [
+    'EOF-token',
+    'ident-token',
+    'function-token',
+    'at-keyword-token',
+    'hash-token',
+    'string-token',
+    'bad-string-token',
+    'url-token',
+    'bad-url-token',
+    'delim-token',
+    'number-token',
+    'percentage-token',
+    'dimension-token',
+    'whitespace-token',
+    'CDO-token',
+    'CDC-token',
+    'colon-token',
+    'semicolon-token',
+    'comma-token',
+    '[-token',
+    ']-token',
+    '(-token',
+    ')-token',
+    '{-token',
+    '}-token',
+    'comment-token'
+];
diff --git a/vanilla/node_modules/css-tree/lib/tokenizer/types.js b/vanilla/node_modules/css-tree/lib/tokenizer/types.js
new file mode 100644
index 0000000..5018569
--- /dev/null
+++ b/vanilla/node_modules/css-tree/lib/tokenizer/types.js
@@ -0,0 +1,28 @@
+// CSS Syntax Module Level 3
+// https://www.w3.org/TR/css-syntax-3/
+export const EOF = 0;                 // <EOF-token>
+export const Ident = 1;               // <ident-token>
+export const Function = 2;            // <function-token>
+export const AtKeyword = 3;           // <at-keyword-token>
+export const Hash = 4;                // <hash-token>
+export const String = 5;              // <string-token>
+export const BadString = 6;           // <bad-string-token>
+export const Url = 7;                 // <url-token>
+export const BadUrl = 8;              // <bad-url-token>
+export const Delim = 9;               // <delim-token>
+export const Number = 10;             // <number-token>
+export const Percentage = 11;         // <percentage-token>
+export const Dimension = 12;          // <dimension-token>
+export const WhiteSpace = 13;         // <whitespace-token>
+export const CDO = 14;                // <CDO-token>
+export const CDC = 15;                // <CDC-token>
+export const Colon = 16;              // <colon-token>     :
+export const Semicolon = 17;          // <semicolon-token> ;
+export const Comma = 18;              // <comma-token>     ,
+export const LeftSquareBracket = 19;  // <[-token>
+export const RightSquareBracket = 20; // <]-token>
+export const LeftParenthesis = 21;    // <(-token>
+export const RightParenthesis = 22;   // <)-token>
+export const LeftCurlyBracket = 23;   // <{-token>
+export const RightCurlyBracket = 24;  // <}-token>
+export const Comment = 25;
diff --git a/vanilla/node_modules/css-tree/lib/tokenizer/utils.js b/vanilla/node_modules/css-tree/lib/tokenizer/utils.js
new file mode 100644
index 0000000..c131ec5
--- /dev/null
+++ b/vanilla/node_modules/css-tree/lib/tokenizer/utils.js
@@ -0,0 +1,254 @@
+import {
+    isDigit,
+    isHexDigit,
+    isUppercaseLetter,
+    isName,
+    isWhiteSpace,
+    isValidEscape
+} from './char-code-definitions.js';
+
+function getCharCode(source, offset) {
+    return offset < source.length ? source.charCodeAt(offset) : 0;
+}
+
+export function getNewlineLength(source, offset, code) {
+    if (code === 13 /* \r */ && getCharCode(source, offset + 1) === 10 /* \n */) {
+        return 2;
+    }
+
+    return 1;
+}
+
+export function cmpChar(testStr, offset, referenceCode) {
+    let code = testStr.charCodeAt(offset);
+
+    // code.toLowerCase() for A..Z
+    if (isUppercaseLetter(code)) {
+        code = code | 32;
+    }
+
+    return code === referenceCode;
+}
+
+export function cmpStr(testStr, start, end, referenceStr) {
+    if (end - start !== referenceStr.length) {
+        return false;
+    }
+
+    if (start < 0 || end > testStr.length) {
+        return false;
+    }
+
+    for (let i = start; i < end; i++) {
+        const referenceCode = referenceStr.charCodeAt(i - start);
+        let testCode = testStr.charCodeAt(i);
+
+        // testCode.toLowerCase() for A..Z
+        if (isUppercaseLetter(testCode)) {
+            testCode = testCode | 32;
+        }
+
+        if (testCode !== referenceCode) {
+            return false;
+        }
+    }
+
+    return true;
+}
+
+export function findWhiteSpaceStart(source, offset) {
+    for (; offset >= 0; offset--) {
+        if (!isWhiteSpace(source.charCodeAt(offset))) {
+            break;
+        }
+    }
+
+    return offset + 1;
+}
+
+export function findWhiteSpaceEnd(source, offset) {
+    for (; offset < source.length; offset++) {
+        if (!isWhiteSpace(source.charCodeAt(offset))) {
+            break;
+        }
+    }
+
+    return offset;
+}
+
+export function findDecimalNumberEnd(source, offset) {
+    for (; offset < source.length; offset++) {
+        if (!isDigit(source.charCodeAt(offset))) {
+            break;
+        }
+    }
+
+    return offset;
+}
+
+// § 4.3.7. Consume an escaped code point
+export function consumeEscaped(source, offset) {
+    // It assumes that the U+005C REVERSE SOLIDUS (\) has already been consumed and
+    // that the next input code point has already been verified to be part of a valid escape.
+    offset += 2;
+
+    // hex digit
+    if (isHexDigit(getCharCode(source, offset - 1))) {
+        // Consume as many hex digits as possible, but no more than 5.
+        // Note that this means 1-6 hex digits have been consumed in total.
+        for (const maxOffset = Math.min(source.length, offset + 5); offset < maxOffset; offset++) {
+            if (!isHexDigit(getCharCode(source, offset))) {
+                break;
+            }
+        }
+
+        // If the next input code point is whitespace, consume it as well.
+        const code = getCharCode(source, offset);
+        if (isWhiteSpace(code)) {
+            offset += getNewlineLength(source, offset, code);
+        }
+    }
+
+    return offset;
+}
+
+// §4.3.11. Consume a name
+// Note: This algorithm does not do the verification of the first few code points that are necessary
+// to ensure the returned code points would constitute an <ident-token>. If that is the intended use,
+// ensure that the stream starts with an identifier before calling this algorithm.
+export function consumeName(source, offset) {
+    // Let result initially be an empty string.
+    // Repeatedly consume the next input code point from the stream:
+    for (; offset < source.length; offset++) {
+        const code = source.charCodeAt(offset);
+
+        // name code point
+        if (isName(code)) {
+            // Append the code point to result.
+            continue;
+        }
+
+        // the stream starts with a valid escape
+        if (isValidEscape(code, getCharCode(source, offset + 1))) {
+            // Consume an escaped code point. Append the returned code point to result.
+            offset = consumeEscaped(source, offset) - 1;
+            continue;
+        }
+
+        // anything else
+        // Reconsume the current input code point. Return result.
+        break;
+    }
+
+    return offset;
+}
+
+// §4.3.12. Consume a number
+export function consumeNumber(source, offset) {
+    let code = source.charCodeAt(offset);
+
+    // 2. If the next input code point is U+002B PLUS SIGN (+) or U+002D HYPHEN-MINUS (-),
+    // consume it and append it to repr.
+    if (code === 0x002B || code === 0x002D) {
+        code = source.charCodeAt(offset += 1);
+    }
+
+    // 3. While the next input code point is a digit, consume it and append it to repr.
+    if (isDigit(code)) {
+        offset = findDecimalNumberEnd(source, offset + 1);
+        code = source.charCodeAt(offset);
+    }
+
+    // 4. If the next 2 input code points are U+002E FULL STOP (.) followed by a digit, then:
+    if (code === 0x002E && isDigit(source.charCodeAt(offset + 1))) {
+        // 4.1 Consume them.
+        // 4.2 Append them to repr.
+        offset += 2;
+
+        // 4.3 Set type to "number".
+        // TODO
+
+        // 4.4 While the next input code point is a digit, consume it and append it to repr.
+
+        offset = findDecimalNumberEnd(source, offset);
+    }
+
+    // 5. If the next 2 or 3 input code points are U+0045 LATIN CAPITAL LETTER E (E)
+    // or U+0065 LATIN SMALL LETTER E (e), ... , followed by a digit, then:
+    if (cmpChar(source, offset, 101 /* e */)) {
+        let sign = 0;
+        code = source.charCodeAt(offset + 1);
+
+        // ... optionally followed by U+002D HYPHEN-MINUS (-) or U+002B PLUS SIGN (+) ...
+        if (code === 0x002D || code === 0x002B) {
+            sign = 1;
+            code = source.charCodeAt(offset + 2);
+        }
+
+        // ... followed by a digit
+        if (isDigit(code)) {
+            // 5.1 Consume them.
+            // 5.2 Append them to repr.
+
+            // 5.3 Set type to "number".
+            // TODO
+
+            // 5.4 While the next input code point is a digit, consume it and append it to repr.
+            offset = findDecimalNumberEnd(source, offset + 1 + sign + 1);
+        }
+    }
+
+    return offset;
+}
+
+// § 4.3.14. Consume the remnants of a bad url
+// ... its sole use is to consume enough of the input stream to reach a recovery point
+// where normal tokenizing can resume.
+export function consumeBadUrlRemnants(source, offset) {
+    // Repeatedly consume the next input code point from the stream:
+    for (; offset < source.length; offset++) {
+        const code = source.charCodeAt(offset);
+
+        // U+0029 RIGHT PARENTHESIS ())
+        // EOF
+        if (code === 0x0029) {
+            // Return.
+            offset++;
+            break;
+        }
+
+        if (isValidEscape(code, getCharCode(source, offset + 1))) {
+            // Consume an escaped code point.
+            // Note: This allows an escaped right parenthesis ("\)") to be encountered
+            // without ending the <bad-url-token>. This is otherwise identical to
+            // the "anything else" clause.
+            offset = consumeEscaped(source, offset);
+        }
+    }
+
+    return offset;
+}
+
+// § 4.3.7. Consume an escaped code point
+// Note: This algorithm assumes that escaped is valid without leading U+005C REVERSE SOLIDUS (\)
+export function decodeEscaped(escaped) {
+    // Single char escaped that's not a hex digit
+    if (escaped.length === 1 && !isHexDigit(escaped.charCodeAt(0))) {
+        return escaped[0];
+    }
+
+    // Interpret the hex digits as a hexadecimal number.
+    let code = parseInt(escaped, 16);
+
+    if (
+        (code === 0) ||                       // If this number is zero,
+        (code >= 0xD800 && code <= 0xDFFF) || // or is for a surrogate,
+        (code > 0x10FFFF)                     // or is greater than the maximum allowed code point
+    ) {
+        // ... return U+FFFD REPLACEMENT CHARACTER
+        code = 0xFFFD;
+    }
+
+    // Otherwise, return the code point with that value.
+    return String.fromCodePoint(code);
+}