A nuanced thing I keep forgetting: When using CSS custom properties and calc() with multiplication, one of the operands _needs_ to have a unit (easy to forget in your fallback values)pic.twitter.com/WwYtFl83Fi
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We can't tell at parse time whether a declaration containing a var() is valid, and it's too expensive to keep around *all* declarations to rewind with, so if substitution would give an invalid value, it instead sets it to initial or inherit.
Oh wow—hmm. Kinda difficult to show the difference between 0 and 0px when the initial style is also 0. 
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