cap

1cap is the cap height of the font the element uses: the distance from the baseline to the top of a flat capital such as H or E. Use it to line things up with capitals. An icon next to a heading with block-size: 1cap is as tall as the capital letters beside it, and the box behind a drop cap can take the height of the letter.

Compare it with ex, which follows lowercase. A mark sized in cap rises above the lowercase letters and stops where the capitals stop. In IBM Plex Sans a capital is about 0.7em tall, so 1cap is roughly 1.35 times its ex here.

The browser takes the cap height from the font’s OS/2 table. The specification says the ascent of the font stands in when the height cannot be determined, and lets a browser estimate it from a capital glyph first. On Linux, Chromium 156 does both: 0.69em from an H glyph that tall, and the ascent for a font with no H glyph. On macOS it does neither: it gave 0.345em for the same H glyph, and 0.711em for a font with no H glyph and an ascent of 0.8em. A font that declares its cap height is not affected. Write cap where the eye measures against the capitals: a vertical rule between two words in small caps, or a box drawn around an all-caps label.