1220353 - vertical-align: middle incorrect when dominant baseline is central (mixed, upright)

Status: NEW

(Core :: Layout: Block and Inline, defect)

Description

[Gérard Talbot]

Unprefixed tests
----------------

http://test.csswg.org/suites/css-writing-modes-3_dev/nightly-unstable/html/vertical-alignment-vrl-022.htm

http://test.csswg.org/suites/css-writing-modes-3_dev/nightly-unstable/html/vertical-alignment-vlr-023.htm

http://test.csswg.org/suites/css-writing-modes-3_dev/nightly-unstable/html/vertical-alignment-vrl-024.htm

http://test.csswg.org/suites/css-writing-modes-3_dev/nightly-unstable/html/vertical-alignment-vlr-025.htm


Expected results
----------------

http://test.csswg.org/suites/css-writing-modes-3_dev/nightly-unstable/html/reference/vertical-alignment-vrl-022-ref.htm


Explanations
------------

Valuable explanation image from Hajime Shiozawa (tests author):

http://lists.w3.org/Archives/Public/public-css-testsuite/2015Oct/att-0001/verticalAlignMiddleWithCentralBaseline.png

When dominant baseline is alphabetic (sideways), then Firefox 45 handles 'vertical-align: middle' correctly: see vertical-alignment-vrl-026 and vertical-alignment-vlr-027 tests at

http://test.csswg.org/suites/css-writing-modes-3_dev/nightly-unstable/html/chapter-7.htm#s7.5


Notes
-----
- Chrome 48.0.2547.0 passes these 4 tests
- I am using Firefox 45.0a1 buildID=20151030030236 and Firefox 45 fails all 4 tests
- I use Linux 3.16.0-66-generic x86_64, Qt: 4.8.6, KDE 4.14.3; Kubuntu (trusty) 14.04.03 LTS
- I've searched for duplicates and did not find any.

Comment 1

[David Baron :dbaron: (⌚️UTC-5, no longer working on Mozilla)]

Looking at these tests makes me wonder if this behavior is desirable.

In particular, the current definition of the alignment point for vertical-align: middle:

Align the vertical midpoint of the box with the baseline of the parent box plus half the x-height of the parent.

seems to assume that the parent has an alphabetic baseline -- that's why it wants to add half the x-height of the parent to get to a "middle" position.

But I don't see why adding half the x-height of the parent is desirable if you're starting from a central baseline.

It seems like a reasonable definition of middle, in a context where the dominant baseline is central, would be either:

• just use the central baseline directly (probably better), or
• use the alphabetic baseline and add half the x-height.

I'm inclined to think we should raise a spec issue here rather than fix these tests, but I'm curious what others think.

Comment 2

[David Baron :dbaron: (⌚️UTC-5, no longer working on Mozilla)]

Actually, I filed https://github.com/w3c/csswg-drafts/issues/4495 so that I don't forget to do so...

Comment 3

[Gérard Talbot]

(...)

1. The primary objective of the "central" vertical align is to align the center of lines. The value "middle" was defined as the center of x-height, which does not work for CJK use cases. So from my point of view, "central" aligns characters to characters (from user perspective, sometimes <img> are characters.)

2. Assuming my understanding of "both use cases exist", "middle" already serves one, so making "central" the same as "middle" looses another use case.

3. I prefer less differences when in horizontal and in vertical, unless it's really needed.
   (...)

https://lists.w3.org/Archives/Public/www-style/2015Jul/0008.html