Relative Font Units
This is a type of length unit which is dependent on the size of a certain character or font size.
Represents the width of the “0” character in the element’s font.
Represents the calculated font-size of the element. If used on the font-size property itself, it represents the inherited font-size of the element.
Represents the x-height of the element’s font. On fonts with the “x” letter, this is generally the height of lowercase letters in the font.
Represents the font-size of the root element (typically <html>). This is different from
em which retrieves its length value from the font size of the element it’s on.