Personally, I can’t see any use for ‘ch’ outside of the context of a monospaced font though, since outside of that context it doesn’t relate to much in the design space.

“ch” is a very handy unit when one is working with CJK entities (or any language that uses fullwidth form, for that matter).

I presume CSS defines ‘ch’ in terms of the width of the zero glyph arbitrarily … Personally, I can’t see any use for ‘ch’ outside of the context of a monospaced font though

There are several variable-width typefaces in which the digits 0 to 9 all have the same width as each other, so this could be useful in laying out numerical data in many fonts.

As Tab notes, this is most useful in dealing with monospaced, ‘fixed width’ fonts, and I presume CSS defines ‘ch’ in terms of the width of the zero glyph arbitrarily so that for any font — at least, any font containing a glyph mapped to the zero character — a ‘ch’ value could be determined. Personally, I can’t see any use for ‘ch’ outside of the context of a monospaced font though, since outside of that context it doesn’t relate to much in the design space.

This page about fontforge has some pertinent info but I’ve yet to find a clear definition.

The ch unit lets you ignore the ratio and just use the width directly, so you know that exactly 40 characters will fit in a “width: 40ch;” box.

this is wrong
> As I understand it the ‘advance width’ is the sum of the width of the glyph itself plus the space before (its left bearing) and the space after (its right bearing)

as is this
> the “advance width” of a glyph is the horizontal space reserved by that glyph for its drawing

this might be true some of the time, depending on engine implementation
> Advance width is how far the pen moves between drawing characters

this might be true, but i don’t see anything requiring that it must be true
> if I were to create a floated element with just a “0” and no whitespace between it and the element’s open and close tags, then the float’s width would be precisely 1ch

“This element should be 10 tʃ wide.”

Advance width is how far the pen moves between drawing characters. This does not correspond directly with how wide the character is, though I’m not 100% certain of CSS’ definition of character box width.

Also, advance width is usually where kerning comes in (strictly speaking, kerning modifies the default advance width of a character). I’m assuming, though, that the CSS ch does not have kerning taken into account, since it’s the default advance width of a 0, not its advance width in relation to another character.