Web-Facing Change PSA: Remove [LegacyNoInterfaceObject] from FontFaceSet IDL

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Dileep Maurya

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6:43 AM (4 hours ago) 6:43 AM

to blin...@chromium.org, Dileep Maurya

Contact emails

dileep...@microsoft.com

Specification

https://drafts.csswg.org/css-font-loading/#fontfaceset

Summary

Chromium's FontFaceSet IDL previously used [LegacyNoInterfaceObject], which hid FontFaceSet as a global property and deleted the constructor property from its prototype. This deviated from the CSS Font Loading spec and differed from Safari and Firefox behavior. This change removes [LegacyNoInterfaceObject] from the FontFaceSet IDL, so FontFaceSet is properly exposed as a global property. Since no constructor() is defined in the IDL, calling new FontFaceSet() from JavaScript now throws TypeError: Illegal constructor, matching the spec. This also fixes the historical.html WPT, which was previously failing in Chromium.

Blink component

Blink>Fonts

Web Feature ID

Font-face-set

Risks

Interoperability and Compatibility

This change removes [LegacyNoInterfaceObject] from FontFaceSet. The IDL retains [Exposed=(Window,Worker)] and defines no constructor. This aligns Chromium with the CSS Font Loading spec (per CSSWG resolution w3c/csswg-drafts#10390), with Firefox and WebKit.

Observable changes for web content:

1. FontFaceSet is exposed as a property of Window and Worker globals: typeof FontFaceSet changes from "undefined" to "function", and "FontFaceSet" in self changes from false to true.

2. new FontFaceSet(...) throws TypeError: Illegal constructor; previously it threw ReferenceError: FontFaceSet is not defined.

3. document.fonts.constructor returns FontFaceSet; previously prototype-chain traversal reached EventTarget because [LegacyNoInterfaceObject] removed FontFaceSet.prototype.constructor. As a consequence, new document.fonts.constructor(...) previously constructed an EventTarget and now throws TypeError: Illegal constructor.

Compatibility risk is low.

No pre-change code path in Chromium produced a FontFaceSet instance: the literal new FontFaceSet(...) threw ReferenceError, and the prototype-walk path new document.fonts.constructor(...) produced an EventTarget. Access to the existing FontFaceSet instance via document.fonts is unchanged. The historical.html WPT and 18 FontFaceSet sub-tests in css/css-font-loading/idlharness.https.html move from failing to passing.

Gecko: Shipped/Shipping

(https://developer.mozilla.org/en-US/docs/Web/API/FontFaceSet)

Firefox has long exposed FontFaceSet as a global without a constructor.

WebKit: Shipped/Shipping

(https://bugs.webkit.org/show_bug.cgi?id=276794)

Web developers: No signals

Security

Low risk.

No new APIs, no new attack surface, and no changes to cross-origin behavior or font loading lifecycle.

Debuggability

No changes to DevTools.

Existing behavior (TypeError for constructor usage) remains unchanged and debuggable.