Intent to prototype and ship: CSS vertical-align shorthand, alignment-baseline, and baseline-shift

[Sajid Anwar]

Summary:
The CSS Inline Layout Module Level 3 specification converts the existing vertical-align longhand property into a shorthand for baseline-source, alignment-baseline, and baseline-shift. This specification unifies CSS inline vertical alignment between HTML and SVG.

We intend to implement this shorthand conversion as well as the new corresponding longhand properties alignment-baseline and baseline-shift (the baseline-source property was implemented previously). This will include support for new keywords in the longhands that were not present on the original vertical-align property (specifically, alphabetic/ideographic/central/mathematical on alignment-baseline and center on baseline-shift). This also includes support for alignment-baseline and baseline-shift for SVG text elements, not previously supported in Gecko.

Bugs:
- vertical-align shorthand: https://bugzilla.mozilla.org/show_bug.cgi?id=1830771
- alignment-baseline implementation: https://bugzilla.mozilla.org/show_bug.cgi?id=1403440
- baseline-shift implementation: https://bugzilla.mozilla.org/show_bug.cgi?id=308338
Specification: https://drafts.csswg.org/css-inline-3/#transverse-alignment
Standards Body: CSSWG
Platform Coverage: All
Preference: None
DevTools bug: N/A
Extensions bug: N/A
Link to standards-positions discussion: N/A
Other browsers:
- vertical-align shorthand — Not supported in Blink or WebKit
- alignment-baseline implementation — Supported for SVG in Blink and WebKit, not HTML
- baseline-shift implementation — Supported for SVG in Blink and WebKit, not HTML
web-platform-tests:
- Multiple existing tests in /css/css-inline for parsing of the alignment-baseline and baseline-shift longhands.
- Some existing tests for vertical-align were updated to reflect the shorthand nature of the property.
- New tests will be added for rendering implementations of the alignment-baseline and baseline-shift properties.