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vertical-align-115/116 invalid

From: L. David Baron <dbaron@dbaron.org>
Date: Sun, 17 Oct 2010 12:46:55 -0700
To: public-css-testsuite@w3.org
Cc: Arron Eicholz <Arron.Eicholz@microsoft.com>
Message-ID: <20101017194655.GA28821@pickering.dbaron.org>
These tests:
http://test.csswg.org/suites/css2.1/20101001/html4/vertical-align-115.htm
http://test.csswg.org/suites/css2.1/20101001/html4/vertical-align-116.htm
http://test.csswg.org/suites/css2.1/20101001/xhtml1/vertical-align-115.xht
http://test.csswg.org/suites/css2.1/20101001/xhtml1/vertical-align-116.xht
are testing for particular alignment of subscripts and superscripts
whose origin I don't understand; it's not the values in the font
metrics.

Gecko implements subscript and superscript alignment using the
metrics of the font, which I think should at least be the preferred
way, although it's not required by the spec.

The Ahem font has a subscript Y offset in its OS2 table of 143 (on a
1000 grid) and a superscript Y offset of 453.  This means that for
48px Ahem, the subscript should have its baseline lowered 6.86px and
a superscript should have its baseline raised 21.74px.  Since the
baseline of Ahem is 1/5 from the bottom (i.e., 9.6px in 48px, 19.2px
in 96px, and 28.8px in 144px), in the superscript testcase the
subscript glyph should have its bottom 16.46px below the line's
baseline (9.6px + 6.86px); in the superscript testcase the
superscript glyph should have its bottom 12.14px above the normal
baseline (21.74 - 9.6).  In contrast, the 96px glyph (in test 115)
should extend 19.2px below the baseline and the 144px glyph (in test
116) should extend 28.8px below the baseline.

This would mean that the values of 'top' to be used in the tests, if
a browser used the font's metrics, and ignoring all the potential
rounding issues from these horribly unround numbers, would be
45.26px (48px - (19.2px - 16.46px)) for test 115 and 55.06px (96px -
(28.8px + 12.14px) for test 116.

In fact, Mozilla on Linux passes the tests if I adjust the 44px to
45px and the 0.6in (57.6px) to 55px.

That said, I think these tests should be removed, since the spec has
no requirement for using the font's metrics for subscript and
superscript offsets.

-David

-- 
L. David Baron                                 http://dbaron.org/
Mozilla Corporation                       http://www.mozilla.com/
Received on Sunday, 17 October 2010 19:47:25 GMT

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