What is the font "EM box / EM unit" and where is it defined

Question

In CSS3 CSS Fonts Module Level 3 (Candidate October 13) , there is a single link to the "EM box", and one to the "EM Unit" in section 2.3. The following is a quote:

[Font-size property] indicates the desired font glyph height. For scalable fonts, font size is the scale factor applied to the font EM element. (Note that some glyphs may extend outside their EM window.) For non-scalable fonts, the font size is converted to absolute units and matched to the declared font size of the font, using the same absolute coordinate space for both consistent values.

What is an “EM box" or corresponding "EM box"?

Browsers (and other user agents) should get this from somewhere, from where?

Answer 1

In a traditional printing house, the space occupied by the capital letter M and the mandatory space around it is always a square. The height or width of the square, "em", is used as the unit of space used by the text. For example, from this definition, one em is the line height of the text with scroll and decode spaces enabled, but without any extra line spacing.

The actual em size in points (if you are talking about paper) or pixels (if you are talking about computers) depends on the font and font size that you have chosen.

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