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Re: [css3-fonts] font-size-adjust auto issue (was: Comments on CSS3 Fonts Module LC)

From: John Daggett <jdaggett@mozilla.com>
Date: Mon, 19 Aug 2013 21:36:15 -0700 (PDT)
To: Vladimir Levantovsky <Vladimir.Levantovsky@monotype.com>
Cc: W3C Style <www-style@w3.org>
Message-ID: <1967783393.4099150.1376973375937.JavaMail.zimbra@mozilla.com>

Vladimir Levantovsky wrote:

> Yes, this is exactly how I understood it and this is why I raised an
> objection to having the value <auto> defined for 'font-size-adjust'
> property. Authors who know what they're doing and who know exactly
> what they want to achieve will use <number> to define the value of
> the property, but I am afraid that many will be mislead by <auto>
> definition that promises to "behave just like a <number>" but, in
> fact, does nothing useful for them. This is exactly the reason I
> suggested to get rid of <auto> and allow the value to be defined
> either as <number> or <none>.

Setting 'font-size-adjust' to *any* value maintains the x-height by
scaling the font size depending on the aspect ratio of the actual font
used.  Using 'auto' merely obviates the need to set an arbitrary
value, it instead uses the aspect ratio of the user agent's default
font (i.e. == initial value of 'font-family').

For example, if Times is the default font and has an aspect ratio of
0.5, then the two style declarations below are equivalent:

  font-size-adjust: auto;
  font-size-adjust: 0.5;

So is your contention here that authors should just use '0.5' or some
arbitrary value rather than using 'auto'?

> So, one will never see the kind of adjustment illustrated by Fig. 19
> when using <auto> - why bother having it defined for this property
> in the first place?

No, any fonts used will be resized so that the x-height matches the
x-height of the default font at the original size.  If Futura has an
aspect value of 0.4, Verdana 0.6 and the default Times 0.5, the
resulting font sizes will be calculated as in the example below:

  font-family: Futura, Verdana;
  font-size-adjust: auto;
  font-size: 20px;

When Futura used:

  used font size = ( 0.5 / 0.4 ) * 20px ==> 25px

When Verdana used:

  used font size = ( 0.5 / 0.6 ) * 20px ==> 16.67px

So authors *will* see the sort of adjustment seen in Fig. 19 when
using the 'auto' value.  The relatively high x-height of Verdana will
be matched with the relatively low x-height of Futura.

If possible, could you explain why you think the 'auto' value "does
nothing useful for [authors]"?  I feel like I'm missing something
here, sorry.


John Daggett
Received on Tuesday, 20 August 2013 04:36:44 UTC

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