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fix(utils): update isShadowRoot to use spec-compliant custom element regex #5059

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fix(utils): update isShadowRoot to use spec-compliant custom element regex #5059

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22 changes: 21 additions & 1 deletion lib/core/utils/is-shadow-root.js
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Original file line number Diff line number Diff line change
Expand Up @@ -18,6 +18,26 @@ const possibleShadowRoots = [
'section',
'span'
];

// Reserved names that match the custom element regex but are not valid custom elements.
// https://html.spec.whatwg.org/multipage/custom-elements.html#valid-custom-element-name
const reservedNames = [
'annotation-xml',
'color-profile',
'font-face',
'font-face-src',
'font-face-uri',
'font-face-format',
'font-face-name',
'missing-glyph'
];

// Spec-compliant PCEN (Potential Custom Element Name) regex.
// Extends the ASCII-only regex to include Unicode characters permitted by the spec.
// https://html.spec.whatwg.org/multipage/custom-elements.html#valid-custom-element-name
const customElementRegex =
/^[a-z](?:[a-z0-9._-]|[\u00B7\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u037D\u037F-\u1FFF\u200C-\u200D\u203F-\u2040\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\uFDF0-\uFFFD])*-(?:[a-z0-9._-]|[\u00B7\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u037D\u037F-\u1FFF\u200C-\u200D\u203F-\u2040\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\uFDF0-\uFFFD])*$/;
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@straker straker Apr 6, 2026 •
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According to the custom element name spec, the name must be a valid element local name, which the spec gives a regex as /^(?:[A-Za-z][^\0\t\n\f\r\u0020/>]*|[:_\u0080-\u{10FFFF}][A-Za-z0-9-.:_\u0080-\u{10FFFF}]*)$/u. I assume we could use that with a modification to match the custom element spec of requiring an alpha lower for the 0th char, no alpha upper chars, and a hyphen. Wouldn't this regex work? /^[a-z][a-z0-9-.:_\u0080-\u{10FFFF}]*-[a-z0-9-.:_\u0080-\u{10FFFF}]*$/u

Tested that customElements.define('c-') is a valid custom element name which means we don't need anything after the hyphen either.

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@straker straker Apr 6, 2026

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I'd also add a link to the valid element local name spec as well

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@bwyard bwyard Apr 6, 2026

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Thanks yeah I'll take a look a little later this evening and read through your suggestions and update the pr to match accordingly


/**
* Test a node to see if it has a spec-conforming shadow root
*
Expand All @@ -29,7 +49,7 @@ function isShadowRoot(node) {
const nodeName = node.nodeName.toLowerCase();
if (
possibleShadowRoots.includes(nodeName) ||
/^[a-z][a-z0-9_.-]*-[a-z0-9_.-]*$/.test(nodeName)
(!reservedNames.includes(nodeName) && customElementRegex.test(nodeName))
) {
return true;
}
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12 changes: 12 additions & 0 deletions test/core/utils/is-shadow-root.js
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Original file line number Diff line number Diff line change
Expand Up @@ -33,6 +33,18 @@ describe('axe.utils.isShadowRoot', function () {
assert.isFalse(isShadowRoot({ nodeName: '0-BUZZ', shadowRoot: {} }));
assert.isFalse(isShadowRoot({ nodeName: '--ELM--', shadowRoot: {} }));
});
it('returns false for reserved custom element names', function () {
assert.isFalse(
isShadowRoot({ nodeName: 'ANNOTATION-XML', shadowRoot: {} })
);
assert.isFalse(isShadowRoot({ nodeName: 'COLOR-PROFILE', shadowRoot: {} }));
assert.isFalse(isShadowRoot({ nodeName: 'FONT-FACE', shadowRoot: {} }));
assert.isFalse(isShadowRoot({ nodeName: 'MISSING-GLYPH', shadowRoot: {} }));
});
it('returns true for Unicode custom element names', function () {
assert.isTrue(isShadowRoot({ nodeName: 'CAF\u00C9-MENU', shadowRoot: {} }));
assert.isTrue(isShadowRoot({ nodeName: 'MATH-\u03A0', shadowRoot: {} }));
});
it('returns false if the native element does not allow shadow DOM', function () {
assert.isFalse(isShadowRoot({ nodeName: 'IFRAME', shadowRoot: {} }));
assert.isFalse(isShadowRoot({ nodeName: 'STRONG', shadowRoot: {} }));
Expand Down