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Re: [css3-fonts] font-size-adjust and em/ex values

From: Simon Sapin <simon.sapin@exyr.org>
Date: Wed, 26 Jun 2013 11:25:25 +0100
Message-ID: <51CAC195.5080904@exyr.org>
To: fantasai <fantasai.lists@inkedblade.net>
CC: www-style@w3.org
Le 25/06/2013 21:14, fantasai a ťcrit :
> EX, CH compute against the used font size of the first available font.
> Since we need to look up the font metrics for this anyway, it seems
> best to be accurate about it. Also these units are really intended to
> match glyph measurements, so they should do so. E.g. Bert's example
> where a colored line matches the ex height of the text, or fitting
> a 10-digit number into a 10ch form input.

This needs a more precise definition of "first available font". Is that 
font family or a font face? How do you find it? As far as I understand, 
metrics belong to a font face, and different faces in the same family 
can have different metrics. (E.g. wider glyph advance in bold vs. normal.)

CR-css3-values-20130404 defines the ch unit as "in the font used to 
render it", which sounds unambiguous enough. (Although it could say 
"font face".)

For ex however you donít have a character to use as input to the font 
matching algorithm, so you might need to define something like that 
algorithm but skipping the step that checks whether a face supports the 
input character. (All faces match that step.)

> EM, I could go either way:
>    * take into account 'font-size-adjust'. Text in the first available
>      font will match 1em measurements in the layout.
>    * just use the computed 'font-size'. Text may end up taller than 1em,
>      which may cause some amount of layout mismatch.
> There are fonts that have ascenders and descenders that go beyond the
> em box, so having the used font size not match the computed EM is maybe
> not such a problem. (If the adjustment is large, though, and the font is
> not designed for overlapping across lines, this could be a problem?)

As an implementer I like the simplicity of the second option, but I 
donít have an opinion on whatís better for authors (which should have 
priority over implementer convenience.)

With the first option you probably need the same font matching as for ex.

Simon Sapin
Received on Wednesday, 26 June 2013 10:25:42 UTC

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