I believe you are correct in your perception of a self-referential problem here. In the absence of an explicit x-height value, a value of 0.5 em would indeed be a reasonable default. (Note that this is not half the cap height: that value itself might average ~ 0.7 em; an aveage x-height might be about 0.7x the cap height.) Cheers, T On Mon, Oct 26, 2009 at 9:59 AM, Philip TAYLOR <P.Taylor@rhul.ac.uk> wrote: > Intrigued by the bug report from Ryan Foster [2], I read > the CSS 3 spec. on units, and read -- to my surprise -- the > following : > > em the font size of the element (or, > to the parent element's font size > if set on the 'font-size' property) > > ex the x-height of the element's font > > Is there any reason why "ex" does not also reference > the parent element's font size if set on the 'font-size' > property, as if "ex" units are used to specify font-size, > there would appear to be a problem of self-reference > (see example below). > > Philip TAYLOR > -------- > Example : > > <HTML> > <HEAD> > <TITLE>Title</TITLE> > </HEAD> > <BODY> > <DIV> > <! -- No explicit font so far --> > <SPAN style="font-size: 0.5em">ABCDabcd</SPAN> > <SPAN style="font-size: 1.0ex">ABCDabcd</SPAN> > </DIV> > </BODY< > </HTML> > > The style declaration for the second span is would > appear to be self-referential, since "the font's x-height" [1] > cannot be known until the font-size has been computed ... > > ** Phil. > -------- > [1] The 'ex' unit is defined by the font's x-height. > The x-height is so called because it is often equal > to the height of the lowercase "x". However, an 'ex' > is defined even for fonts that don't contain an "x". > Should we say that ex is 0.5em if no better value exists? > -------- > [2] Ryan Foster wrote: > >> When set to CSS3, the CSS Validator states: >> >> Value Error : min-width >> >> <http://jigsaw.w3.org/css-validator/nullvisudet.html#propdef-min-width> >> Unknown dimension 15ch >> >> While the CSS3 Values and Units <http://www.w3.org/TR/css3-values/> draft >> specification states <http://www.w3.org/TR/css3-values/#relative0>: >> >> ch: The width of the "0" (ZERO, U+0030) glyph found in the font for >> the font size used to render. If the "0" glyph is not found in the >> font, the average character width may be used. How is the "average >> character width" found? >> >> The Validator should mark "ch" as a valid unit when set to CSS3. The same >> error occurs in the development version >> (http://qa-dev.w3.org:8001/css-validator/). > >Received on Monday, 26 October 2009 17:24:23 UTC
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