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

Re: Units, font sizing, and zoom suggestion for CSS 3

From: Ian Hickson <py8ieh@bath.ac.uk>
Date: Tue, 25 Jan 2000 09:04:44 +0000 (GMT)
To: Erik van der Poel <erik@netscape.com>
cc: Karlsson Kent - keka <keka@im.se>, www-style <www-style@w3.org>
Message-ID: <Pine.GSO.4.04.10001250851310.23636-100000@mary.bath.ac.uk>

[Note. I'm partly playing Devil's Advocate here -- merely highlighting
the opposing view points. I no longer really know what my own personal
opinions on the issues are. :-/ ]

On Mon, 24 Jan 2000, Erik van der Poel wrote:
>> I strongly dislike the idea that "em" should mean different things
>> in two major modern digital typesetting contexts. TeX, and its
>> successor Omega, are not going away as far as I can see. "em" has
>> in several, but not all, typographic traditions *into modern time*
>> meant "width of M", at least for fonts suitable for running text.
>> That's what it means also in the very widely used TeX system. Let's
>> stick to that.
> I'm beginning to warm up to Kent's idea here. Since the current
> definition of "em" in CSS is so vague, 

The definition of _em_ is _not_ vague. Section 4.3.2:

#  * em: the 'font-size' of the relevant font

The only minor vagueness is whether one should use the specified,
computed, or actual value of font-size. I tend to believe we should
use the actual value of font-size for the first available font in the
font-family list, but this is not (AFAIK) specified in the spec.

> and since the implementations don't appear to implement it
> consistently,

I think 'em' is implemented pretty well and consistently. Maybe
font-size, and certainly line-height and vertical-align, but the 'em'
unit is generally well supported. (I have a test case somewhere...)

> and since CSS is still in its infancy in terms of volume of data on
> the Net and in terms of removing vagueness in the spec, I think it's
> not too late to "change" (clarify) the definition of "em". There
> aren't many documents or style sheets out there using "em", for
> various reasons.

Not many compared to the billions of document actually on the web, no,
but in absolute terms we are still talking tens to hundreds of
thousands, I expect. Maybe even millions. That is still a _lot_ of
documents. (Sorry, no reference...)

> Also, when you pronounce the unit "em", it sounds like the letter M.

If we stick to the definition of font-size as given by Eric in his
document (which IIRC the WG has agreed is correct),


...then the height of the em square is equal to the font-size. Since
'em' is equal to the font-size too, that means that 1em is the height
of the em square of the element's first-choice font.

That makes sense to me, and explains why it is spelt 'em' not 'M'.

>> (For most other scripts, a suitable similar measure should not be
>> too hard to find, I think. Perhaps Arabic/Mongolian are hard for
>> this.)
> We need to define what font-size means for all of those scripts
> anyway.

Do they not have an em square?

Ian Hickson                            ("`-''-/").___..--''"`-._   
http://www.bath.ac.uk/%7Epy8ieh/        `6_ 6  )   `-.  (     ).`-.__.`)
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Member, Mozilla Quality Assurance     _..`--'_..-_/  /--'_.' ,'
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Received on Tuesday, 25 January 2000 04:04:54 UTC

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