Reifying the result gets quite ad-hoc. Firstly because "parse a
component value" produces a ComponentValue, not a full StyleValue like
we need for math functions. And second, because not all math functions
can be reified as a CSSNumericValue:
Besides the fact that I haven't implemented CalculatedStyleValue
reification at all yet, there are a lot of math functions with no
corresponding CSSMathValue in the spec yet. If the calculation tree
contains any of those, the best we can do is reify as a CSSStyleValue,
and that isn't a valid return value from CSSNumericValue.parse(). So, I
made us throw a SyntaxError in those cases. This seems to match
Chrome's behaviour. Spec issue:
https://github.com/w3c/css-houdini-drafts/issues/1090
