Remove unit only from <length>

Spec does not mentioned it can be removed or not, but unit-less <angle>,
<frequency>, <time>, and <resolution> are invalid on Firefox:
  - https://developer.mozilla.org/en-US/docs/Web/CSS/angle
  - https://developer.mozilla.org/en-US/docs/Web/CSS/frequency
  - https://developer.mozilla.org/en-US/docs/Web/CSS/time
  - https://developer.mozilla.org/en-US/docs/Web/CSS/resolution
Merge branch 'keep-angle-freq-resolution'