add support for dimensions relative to font size

[mockersf]

What problem does this solve or what need does it fill?

To help with responsive design, dimensions relative to font size are useful

What solution would you like?

Add support for em and rem. Additionally, ex and ch exist but are less useful

What alternative(s) have you considered?

None

Additional context

Unit	Description
em	Relative to the font-size of the element (2em means 2 times the size of the current font)
rem	Relative to font-size of the root element
ex	Relative to the x-height of the current font (rarely used)
ch	Relative to the width of the "0" (zero)

https://www.w3schools.com/cssref/css_units.php