Summary: For historical/legacy reasons, fonts may contain multiple, conflicting versions of key metrics such as line ascent and descent, and user agents/platforms may rely on different fields in the font. This results in inconsistencies in line spacing, baseline alignment, etc.

To enable authors to achieve more consistent layout, the CSS WG has added descriptors to the @font-face rule that override whatever value the browser would use from the font resource, and provide an explicit author-provided value instead.

This can be used to harmonize line spacing behavior across browsers and platforms, or across different fonts where the designers may have made incompatible choices regarding the line metrics.



Platform coverage: All

Preference: layout.css.font-metrics-overrides.enabled

DevTools bug: None needed. (More generally, it would be awesome to have a @font-face rule inspector in DevTools -- distinct from the existing font *properties* inspector; such an inspector would expose this along with the other @font-face descriptors. But that's not specific to these new descriptors.)

Other browsers: Already shipping in Chrome.

web-platform-tests: There are a few tests in /css/css-fonts/, which will pass in Firefox once this is implemented.
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