Intent to Ship: text-box-trim / text-box-edge

[Sajid Anwar]

As of Firefox 154, I intend to turn the text-box-trim / text-box-edge (and associated text-box shorthand) CSS properties on by default on all platforms. These have been developed behind the layout.css.text-box.enabled preference. 

These features have been shipped in other browsers since Chrome 138 and Safari 26.2.  Notable differences with Blink and/or WebKit (see the WPT test suite for full results):

• Gecko supports the ideographic and ideographic-ink edges, which are not supported by Blink or WebKit.
• Gecko supports trimming of multi-column containers, which is supported by Blink but not by WebKit.
• Gecko supports trimming of inline boxes, which is supported by WebKit but not by Blink.
• Gecko does not support trimming of line-clamp-ed boxes (Bug 2043012), which is also not supported by WebKit but which is supported by Blink.

Bugs:

- Bug 1816038 - [css-inline-3] Implement `text-box-trim` (formerly `leading-trim`) - Main bug.

- Bug 2050141 - [text-box-trim] Set layout.css.text-box.enabled for all users - Bug to launch feature.

Standard:

https://drafts.csswg.org/css-inline-3/#text-box-trim

Intent to Prototype Thread:

https://groups.google.com/a/mozilla.org/g/dev-platform/c/Ys0yVWrjo_E

Additional Links:

Open questions to clarify behavior of implementation or tests:

- w3c/csswg-drafts#14136 ("[css-inline-3] Clarification on text-box-trim propagation and independent block formatting contexts")

- web-platform-tests/wpt#61079 ("[css-inline-3] Is text-box-trim-half-leading-block-box-003.html incorrectly counting phantom line boxes?")