fix(lib): allow CSSStyleValue in Keyframe index signature

[creazyfrog]

Summary

Fixes #63325

The Web Animations API is compatible with the CSS Typed Object Model — properties like transform can accept CSSTransformValue objects (CSSRotate, CSSTranslate, etc.) at runtime in modern browsers. However, the Keyframe index signature only allowed string | number | null | undefined, causing a type error on valid code:

element.animate([
    { transform: 'rotate(0deg)' },
    { transform: new CSSRotate(45, 0, 0, CSSNumericValue.parse('90deg')) } // ❌ TS error before fix
], { duration: 500 })

Fix

Add CSSStyleValue — the common base class for all CSS Typed OM types — to the index signature:

 interface Keyframe {
     composite?: CompositeOperationOrAuto;
     easing?: string;
     offset?: number | null;
-    [property: string]: string | number | null | undefined;
+    [property: string]: string | number | null | undefined | CSSStyleValue;
 }

Using the base class rather than enumerating individual subclasses (CSSTransformValue, CSSNumericValue, CSSKeywordValue, …) keeps the change minimal and future-proof — any CSS Typed OM subclass is automatically covered.

Spec reference

The Web Animations spec and MDN element.animate() confirm that CSS Typed OM values are valid for keyframe properties in supporting browsers.

[typescript-bot]

It looks like you've sent a pull request to update some generated declaration files related to the DOM. These files aren't meant to be edited by hand, as they are synchronized with files in the TypeScript-DOM-lib-generator repository. You can read more here. For house-keeping purposes, this pull request will be closed.