style: Don't incorrectly clamp values in calc that might not be only lengths.
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Expressions with percentages may be negative or positive at computed value time.
So, we can only clamp lengths at computed value time, which is what the other
browsers do.
Handle specialized serialization of <position> in basic shapes
Fixes#13083
We temporarily broke basic-shape serialization in #13042 when 4-value positions were implemented, since I didn't want to increase the scope of that PR too much.
This fixes it.
r? @SimonSapin
cc @canaltinova
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