When we resolved in #4934 to introduce overflow-clip-margin, we did not discuss whether it should take a <length> or a <length-percentage>.

margin takes a <length-percentage>, but the new spec text defines overflow-clip-margin to take a <length>. We didn't discuss that point. On the one hand, analogy with margin would suggest <length-percentage>. On the other hand, overflow-clip-margin is mainly expected to be used to account for things like shadows and outlines, which do not take percentages. I'm leaning towards leaving it as a <length>, but not 100% sure, and we didn't discuss.