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[css-text-decor-3] drop 'text-underline-position: alphabetic'

From: L. David Baron <dbaron@dbaron.org>
Date: Sun, 24 Mar 2013 20:58:47 -0700
To: www-style@w3.org
Message-ID: <20130325035847.GA29444@crum.dbaron.org>
http://dev.w3.org/csswg/css-text-decor-3/#text-underline-position-property
describes the acceptable values of text-underline position as:

  # Value:    auto | alphabetic | [ under || [ left | right ] ]
  # Initial:  auto 

I propose that the 'alphabetic' value be dropped, for the following
reasons.


The 'auto' value is, I presume, intended to reflect the existing
behavior.  The reality of at least some existing implementations is
that they ask the first available font [1] for its underline position
and thickness, and use that.  (There may be some implementations
that use position but not thickness.)


The 'alphabetic' value, on the other hand, seems to require
implementations that ask for font metrics to do a bunch of extra
work to figure out whether the position suggested by the font is
relative to the alphabetic baseline.  It's not clear to me how an
implementation would do this, though I'm not a font expert.
Alternatively, a conformant implementation could ignore font metrics
entirely and do its own calculation, but that seems likely to result
in a less desirable rendering since the font's preferred underline
metrics are ignored.

It's also not clear to me what the use cases for the 'alphabetic'
value are.  I believe the current behavior (using font metrics)
generally produces good results, and when it doesn't, that's the
fault of the font.  If there is a particular behavior that is in
general more desirable than using font metrics, the specification
should require that behavior.  If there are a particular set of
languages that require this value, that should be explained in the
specification (and I'd be willing to reconsider this proposal).  But
barring those, I think the 'alphabetic' value should be dropped.

-David

[1] That is, the font that results from the font matching algorithm
if the character input and the checks for whether the face has a
glyph for a particular character are removed.  (We ought to have a
term for this.)

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                           http://www.mozilla.org/   𝄂
Received on Monday, 25 March 2013 03:59:10 GMT

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