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Re: [css3-fonts] alternative to font-size-adjust:auto [was Re: Agenda conf call 21-aug-2013]

From: John Daggett <jdaggett@mozilla.com>
Date: Sun, 25 Aug 2013 23:12:05 -0700 (PDT)
To: Vladimir Levantovsky <Vladimir.Levantovsky@monotype.com>
Cc: www-style@w3.org
Message-ID: <1932317311.5063346.1377497525911.JavaMail.zimbra@mozilla.com>
Vladimir Levantovsky wrote:

> Thank you for your replies and the detailed comments. Based on the
> specific details presented during this email discussion I am still
> not convinced that <auto> value is useful and practical. It seemed
> that, to the contrary, in most of the cases the rendering results
> will be different on different platforms, and that the applied
> adjustments will most likely adversely affect the text legibility.

Yes, the size will be different but the x-height of the used font will
always match the x-height of the local default font.  That's what Tab
was emphasizing. Authors who want the x-height to remain constant
across platforms can use a generic, fixed value (e.g. 0.5).

> My initial suggestion to have only two possible values defined
> (<none> and <number>, with <none> being the default) seems to be a
> good alternative option (and a simple fix):
> - no harm done if nothing is specified,
> - authors who know what they are doing will define a specific number
>   based on their 'first choice' font,
> - consistent results on different platforms,

Ok, so you feel the 'auto' value should be dropped.

> I also believe that the scaling behavior needs to be described in the
> spec. In particular, it needs to clarify that while font size may be
> adjusted to specified aspect value, the line spacing of text needs to
> be calculated based on the declared font-size and that in some cases
> the results of the font-size-adjust may cause either a collision
> between ascenders and descenders or clipping (e.g. when a font with
> very small original aspect value is adjusted to specified, larger
> value of font-size-adjust property

The 'font-size' and 'line-height' properties control font size and
leading in CSS.  They are not directly linked but authors can use
font-relative units [1] to make the line-height a function of the
font-size:

  font-size:   200%;
  line-height: 1.2em;  /* == computed font size x 1.2 */

Back in June the WG discussed whether the 'em' unit should be tied to
the computed value *after* 'font-size-adjust' is applied. The
conclusion of that discussion was that it should not for the 'em'
unit. The behavior you're asking for would be done in CSS via the 'em'
unit, so I think you're basically making the argument that the 'em'
*should* follow the effective font size after 'font-size-adjust'
adjustments have been applied. I think your concern is that if
'font-size-adjust' doesn't affect the 'em' unit value used for
'line-height', then collisions may potentially occur.

I think in general the variations across platforms are relatively
minor (+/- 10%) so I'm not convinced that collisions would result in
practice.  But I do realize it's possible, especially if the author is
keeping the leading very tight.

I guess I'd like to know if you think this can be handled by a note in
the spec or whether you think the defined behavior needs to be
changed, for example by adjusting the 'em' size based on
'font-size-adjust'.

Regards,

John Daggett

[1] http://www.w3.org/TR/css3-values/#font-relative-lengths
[2] http://lists.w3.org/Archives/Public/www-style/2013Jul/0092.html
Received on Monday, 26 August 2013 06:12:34 UTC

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