5.1.1 Font-relative lengths: the em, ex, ch, rem units

Chapter 5 - Distance Units: the type - 5.1.1 Font-relative lengths: the em, ex, ch, rem units

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ch-unit-001
support for the ch unit
The ch unit is equal to the used advance measure of the 0 (ZERO, U+0030) glyph found in the font used to render it.
ch-unit-002
the ch unit in vertical orientation
In vertical upright, the ch unit is equal to the used vertical advance measure of the 0 (ZERO, U+0030) glyph found in the font used to render it.
ch-unit-003
support for the ch unit
In vertical mixed, The ch unit is equal to the used horizontal advance measure of the 0 (ZERO, U+0030) glyph found in the font used to render it.
ch-unit-004
support for the ch unit
In vertical sideways, The ch unit is equal to the used horizontal advance measure of the 0 (ZERO, U+0030) glyph found in the font used to render it.