The folks at freetype.org have gotten "advance measure" (also known as simply "advance") correct -- It depends on the horizontal or vertical layout orientation.

The folks at freetype.org have gotten "advance measure" (also known as simply "advance") correct -- It depends on the horizontal or vertical layout orientation. Unfortunately, it seems the mozilla folks may have implemented the CSS3 'ch' unit incorrectly: http://www.w3.org/TR/css3-values/#ch-unit http://www.w3.org/TR/css3-values/#ch-unit ch unit Equal to the used advance measure of the "0" (ZERO, U+0030) glyph found in the font used to render it. https://developer.mozilla.org/en-US/docs/Web/CSS/length https://developer.mozilla.org/en-US/docs/Web/CSS/length ch This unit represents the width, or more precisely the advance measure, of the glyph '0' (zero, the Unicode character U+0030) in the element's font. If you change the layout orientation of text from horizontal to vertical, the "advance measure" will change. If the "advance measure" remains constant between the two, then vertically orientation would result in characters (glyphs) overwriting each other. The above is just the mozilla docs so I really need to check the mozilla source code to make sure.