em Relative to the font size, i.e. 1.5em will be 50% larger than the base computed font size of its parent. (Historically, the height of the capital letter “M”). ex Heuristic to determine whether to use the x-height, a letter “x”, or .5em in the current computed font size of the element. cap Height of the capital letters in the current computed font size of the element. ch Average character advance of a narrow glyph in the element’s font (represented by the “0” glyph). ic Average character advance of a full width glyph in the element’s font, as represented by the “水” (CJK water ideograph, U+6C34) glyph. rem Font size of the root element (default is 16px). lh Line height of the element. rlh Line height of the root element.