W3C home > Mailing lists > Public > www-style@w3.org > February 2000

Re: What's an em

From: Erik van der Poel <erik@netscape.com>
Date: Tue, 01 Feb 2000 14:03:04 -0800
Message-ID: <38975818.8583EE1F@netscape.com>
To: Karlsson Kent - keka <keka@im.se>
CC: www-style@w3.org, www-font@w3.org
Karlsson Kent - keka wrote:
> It is especially important to get this done correctly for the
> web, where for presentation one does not know the EXACT font(s)
> used, but a font substitution is done.

Yes, this is an important point. There are at least 2 alternative
solutions to your "problem":

(1) Don't change the CSS spec, except perhaps to clarify that CSS's
font-size and em use the TrueType definition of "em" (in the unitsPerEm
field). This would require all font vendors to update their fonts if
they want users to be able to use them on the Web, where unpredictable
substitution occurs. Many vendors may not care about the Web, and may
leave their old fonts as they are. Or they may come up with new designs
specifically for the Web. Doesn't Adobe have particular fonts for the
Web, even with the word "Web" in their names?

(2) Don't change the fonts, but change the CSS spec and the CSS
implementations to use a different definition of font-size and em, e.g.
the p height. This way, the fonts can continue to use some ad hoc
interpretation of TrueType's "unitsPerEm" field, and CSS can compensate
for this randomness by specifying that the font-size is based on
particular actual glyph sizes, e.g. median of x-heights if font-size is
set for ex-height.

The big disadvantage of (2) is that we would be making an incompatible
change to CSS, and we already have a number of CSS implementations out
there, actually used by users. (Unless we define font-size in a clever
way, so that old implementations and new implementations use the
TrueType em when font-size is specified the normal way, and new
implementations use p, cap or ex when font-size is specified in one of
your new ways.)

> (Opentype?) focuses on a measure
> (descender to ascender height) that is *NOT* of major interest
> to the font USER.

What, exactly, is the OpenType "ascender" and "descender"? Does the
ascender refer to the tops of the letters bdfhkl? Or does ascender refer
to the accents above the capital letters?

> H-height -- (nominal) cap height, useful for specifying consistent
>                 size, even under font substitution, for ideographic text
>                 as well as Latin/Greek/Cyrillic cap only headings.

Ideographic text (e.g. Japanese) has lots of strokes (complexity). You
need more pixels to display Japanese than English. So, linking East
Asian text size to H-height doesn't seem like a good idea. How about
linking it to p height? (This is just a question. It does not imply
that I favor your proposal.)

> For best backwards compatibility, I've suggested
> that the p-height is what is set by default.

I disagree. The TrueType em height should be set by default, as it is

Received on Tuesday, 1 February 2000 17:06:19 UTC

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