Going beyond pixels and (r)ems in CSS - Relative length units based on font - iO tech_hub
Briefly

The cap unit is useful for creating layouts that are scale-independent and responsive to different fonts and font sizes.
The ch unit represents the advanced measure of the character 0 (zero) in the font used to render it. It is an absolute length unit, though it scales with the font-size of the current element.
Read at iO tech_hub
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