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

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

From: David Perrell <davidp@earthlink.net>
Date: Fri, 21 Jan 2000 18:43:21 -0800
To: www-style <www-style@w3.org>
Message-id: <01fa01bf6482$729114e0$15a8a8c0@davidp>
"Erik van der Poel" wrote:
>   This property describes the size of the font when set solid.
> I want to get rid of the words "when set solid", and I want to clarify
> the font-size definition. I think the words "em square" would have to
> appear somewhere in that definition. Who can come up with good wording?

I agree that terminology doesn't seem appropriate. But I don't see a problem
with: 'Font-size is the vertical height allotted for a horizontal line of
text when line-height is 1 or 100%.' I like your paragraph

>   This property corresponds directly to the em square [link to
>   definition of em square]. Outline fonts are scaled to make
>   their em square height coincide with the requested font-size.
>   Bitmap fonts have a size property expressed as a number of
>   pixels. UAs may search for the bitmap font with the size
>   closest to the requested font-size. See also the font matching
>   algorithm [link to font-size part of font matching algorithm].

except that the property does not correspond directly to an em square. CSS2
definition of em square is misleading. Font metrics are not necessarily
defined in terms of the em square. Type 1 fonts are defined in terms of
units in an abstract coordinate system (character space). The relationship
of character space to rendering space is determined by the number of
characterspaceunits per renderspaceunit. For Type 1 fonts, 1000
characterspaceunits == font-size == 1em. The position of an em square in
character space is not defined.

Em is simply shorthand for "current font size". Its value is that you don't
need to know absolute font size in order to specify relative dimensions.

Regarding TrueType's "recommended leading," this makes little sense unless
an "at absolute size" field is included as well. Body text is often set with
expansive line-spacing while large headlines are often set with negative
line-spacing ('loose' headlines can lose impact and integrity). The only
reasonable use for this value would be to compute the meaning of
"line-height: normal".

> ...
> No, font-size is equal to one line-height only when line-height is 1.
> When line-height is 1.2, font-size is not equal to one line-height.

Yes, that was a boo-boo.

> ...
> I suggested line spacing, font spacing and vertical spacing. We should
> choose one of these terms, and stick to it.

Letterspace and word space are common terms. Linespace seems most intuitive.

David Perrell
Received on Friday, 21 January 2000 21:49:17 UTC

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