If I use rem as a length unit, I get an "invalid value" error:

ERROR Property: Invalid value for "CSS Backgrounds and Borders Module Level 3" property: 0.75rem [1:1: border-radius]

On investigation, it seems that in https://github.com/jaraco/cssutils/blob/main/cssutils/profiles.py, rem is not supported as a unit:
'length': r'0|{num}(em|ex|px|in|cm|mm|pt|pc)',

Is this something that could be added? I'm not particularly familiar with this module as it's a subdependency so I don't know if it would take more than just adding it to the pattern.