ch
The width of the digit zero — forty of them make a good line of text.
One ch is the horizontal advance of the digit zero, 0, in the font and size where the measure is used. In monospaced fonts it is the exact width of any character; elsewhere, an honest approximation of the average. When the browser cannot measure the zero, the standard decrees 0.5em.
It is the right choice for widths that are text: max-width: 60ch holds a paragraph inside the reading-comfort zone, and a form field with width: 8ch grows and shrinks with its own font — the measure follows the content, not the other way around.
This paragraph is pinned to a max-width of 40ch: each line carries at most the equivalent of forty zeros of the font you are reading it in right now.
p {
max-width: 40ch;
}