I'm giving up on this topic.
After reading the specs numerous times, I'm realizing that the specs themselves are circular. I
feel like I'm trying to find out if the chicken or the egg came first.
On ems and exes:
http://www.w3.org/TR/2011/REC-CSS2-20110607/syndata.html#length-units :" em: the 'font-size' of the
relevant font. It may be used for vertical or horizontal measurement. (This unit is also sometimes
called the quad-width in typographic texts.)"
"ex: the 'x-height' of the relevant font"
"If reliable font metrics are not available, UAs may determine the x-height from the height of a
lowercase glyph. "
http://www.w3.org/TR/2011/REC-CSS2-20110607/fonts.html#propdef-font-size: On all other properties,
'em' and 'ex' length values refer to the computed font size of the current element.
em = font-size = em = font-size = em = font-size === paceaux wants to finger paint
On Jan 25, 2012, at 3:04 AM, Ghodmode wrote:
> I think we're going around in circles.
> Here's my existing experiment page:
> I'm going to do another one with more information.
> It's a square block, 1em wide and tall, with a lowercase 'm' inside
> block and the numbers it comes up with match what Firebug & friends
> say for the computed height and width.
> It shows that an em is as wide as it is tall, but it's not the size of
> the letter 'm'.
> Since the block's width is the same as its height, that shows that em
> is both a horizontal measurement and a vertical measurement. However,
> the letter doesn't fit, so an em isn't based on the size of a letter
> in the font specified... at least not the letter 'm'.
> more inline ...
> On Wed, Jan 25, 2012 at 6:28 AM, Tim Climis [EMAIL-REMOVED]> wrote:
>>> From my this, it really visually appears as if the em is not an "m" or an "M" in
>>> even the most plain typeface. That's when the text is centered. If it's left or
>>> right aligned, you can fit in two more "m".
>> As has been discussed before in this thread, em is not a horizontal measure. It is a vertical
measure, and is defined as the size of the font.
> But a 1em block is a square. It's the same size vertically as it is
> horizontally. How can it be only a vertical measure, or only a
> horizontal measure?
> The problem is, it's not a measure of anything. It's relative to the
> font size, but none of the letters in the font are necessarily 1em
> tall or wide. This is the part I didn't understand before.
>> Directly from the CSS 1 spec (just to show that it's always been defined this way - at least in
CSS) "ems, the height of the element's font" http://www.w3.org/TR/CSS1/#units
> You copied that from the comment in one of the example code blocks,
> not the actual description of the unit. It still leaves the question:
> How big is that?
> What it actually says is "The relative units 'em' and 'ex' are
> relative to the font size of the element itself." It doesn't go on to
> say how they relate to the font size.
>> The CSS 2.1 spec gets more precise, particularly in regard to x-height.
> The CSS 2.1 spec does make it much clearer by linking to the font-size
> property definition. So, the 'em' is the font-size. But then it says
> "The 'em' unit is equal to the computed value of the 'font-size'
> property of the element on which it is used." That makes me ask
> "Huh?! How is it computed? How big is an 'em'?!" If they just took
> that word "computed" out of there, it would have been easier for me to
> That's perfectly clear to some of you on this list?
> What I think it should say is that 1em is equal to the element's
> font-size. If the font-size isn't defined, the size of the em is
> equal to the user agent's default font size.
> Vince Aggrippino
> a.k.a. Ghodmode
> css-discuss [EMAIL-REMOVED]]
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