Bug 1860319 [wpt PR 42671] - WebKit export of https://bugs.webkit.org/show_bug.cgi...

Bug 1860319 [wpt PR 42671] - WebKit export of https://bugs.webkit.org/show_bug.cgi?id=263461, a=testonly

Automatic update from web-platform-tests
WebKit export of https://bugs.webkit.org/show_bug.cgi?id=263461 (#42671)

To synthesize a baseline, the spec says the following: "synthesize the
alphabetic baseline from the line-under line, and the central baseline
by averaging the positions of the two edges or lines."

When the writing mode is vertical-lr and text-orientation is sideways,
we must synthesize the alphabetic baseline. This means that the baseline
for the item in this writing-mode is its block-start and not the
block-end like in vertical-rl writing mode.

We can first determine the baseline that we are synthesizing according
to the logic in: https://drafts.csswg.org/css-inline-3/#dominant-baseline-property
If the baseline type is alphabetic and we are in a vertical-lr writing
mode according to the formatting context root, then we can return 0_lu
for the synthesized baseline.
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wpt-commits: 242a092647045bb39b4583f741564f5a694c1f37
wpt-pr: 42671