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Re: line-height: normal and multiple descendant font sizes

From: Glenn Adams <glenn@skynav.com>
Date: Sun, 20 Oct 2013 12:06:39 -0600
Message-ID: <CACQ=j+etUV1PNHwA0RJVvZ=EsXcpNOMhg5FzRwAy+1i9yYwprg@mail.gmail.com>
To: "L. David Baron" <dbaron@dbaron.org>
Cc: W3C Style <www-style@w3.org>
On Sun, Oct 20, 2013 at 11:46 AM, Glenn Adams <glenn@skynav.com> wrote:

>
>
> On Sat, Oct 19, 2013 at 2:18 AM, L. David Baron <dbaron@dbaron.org> wrote:
>
>> On Thursday 2013-10-17 10:52 -0600, Glenn Adams wrote:
>> > In CSS2.1, we have the following in Section 10.8.1:
>> >
>> > "When an element contains text that is rendered in more than one font,
>> user
>> > agents may determine the 'normal'
>> > 'line-height'<
>> http://www.w3.org/TR/CSS2/visudet.html#propdef-line-height>
>> > value
>> > according to the largest font size."
>> >
>> > I'm curious if any UA actually implements this. I have tested Chrome,
>> > Opera, and Safari UAs, and none of these UAs seems to use the largest
>> font
>> > size (of descendant fonts).
>> >
>> > My test consisted of the following fragment:
>> >
>> > <p style="line-height: normal">
>> > <span style="font-size: 12pt; border: 1px solid #C0C0C0">X</span></br>
>> > <span style="font-size: 36pt; border: 1px solid #C0C0C0">X</span></br>
>> > <span style="font-size: 18pt; border: 1px solid #C0C0C0">X</span></br>
>> > </p>
>> >
>> > If a UA implements the above language, then I would expect that each of
>> the
>> > three lines be assigned a single line height (of 36pt), where the
>> leading
>> > on each line is computed according to the following language:
>> >
>> > "Still for each glyph, determine the leading L to add, where L =
>> > 'line-height' <
>> http://www.w3.org/TR/CSS2/visudet.html#propdef-line-height>
>> >  - AD. Half the leading is added above A and the other half below D,
>> giving
>> > the glyph and its leading a total height above the baseline of A' = A +
>> L/2
>> > and a total depth of D' = D + L/2."
>> >
>> > Also, I should note that "the largest font size" is ambiguous in the
>> above
>> > language, since it might mean:
>> >
>> >    1. largest font size of descendant non-replaced elements, i.e.,
>> >    descendants only;
>> >    2. largest font size of descendant non-replaced elements and element
>> (on
>> >    which line-height of 'normal' is specified), i.e., descendants and
>> self.
>>
>> This sentence is not intended to refer to text inside descendant
>> elements, only text in the element itself.  That could certainly be
>> clearer.
>>
>> What it's referring to is that the font matching algorithm is
>> per-character, so different fonts might be used for different
>> characters within the element.  These fonts might have different
>> sizes (due to unavailability of some font sizes with bitmap fonts)
>> or might have font metrics that suggest different normal line
>> heights, or (a case the spec doesn't cover) might have font metrics
>> that suggest that normal line height has different positions
>> relative to the baseline.
>>
>
> I've did some research on Blink/Webkit, and both resolve line-height
> 'normal' on a block element to a single value that is used by the
> subsequent process of computing each line box's height:
>
> resolvedLineHeight = *ascent* + *descent* + *leading*
>
> where the dependent variables are based only on metrics obtained from the
> font associated with the single computedFontFamily associated with the
> block.
>

I suppose *computedFontFamily* is the wrong term here. I'm not sure we have
a good term for the actual font used for a given element, i.e., neither *used
value* or *actual value* of the font-family property seem adequate. [I'm
not sure if we define these latter two anywhere for font-family for that
matter.]


>
> The value of *leading* here is based on a per-platform-font metric, e.g.,
> on Windows the TEXTMETRICS.tmExternalLeading field, on Mac,
> CTGetFontLeading(), etc.
>
> Does FF use this same strategy? If all UAs use the same strategy, I wonder
> (again) if we should remove the language about "may [use] largest font
> size"?
>
>
>
>>
>> As Alan points out, the test isn't testing the quoted text.
>>
>> -David
>>
>> --
>> 𝄞   L. David Baron                         http://dbaron.org/   𝄂
>> 𝄢   Mozilla                          https://www.mozilla.org/   𝄂
>>              Before I built a wall I'd ask to know
>>              What I was walling in or walling out,
>>              And to whom I was like to give offense.
>>                - Robert Frost, Mending Wall (1914)
>>
>
>
Received on Sunday, 20 October 2013 18:07:27 UTC

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