[css-properties-values-api] Clarify whether lengths using viewport units are "computationally independent"

[heycam]

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https://drafts.css-houdini.org/css-properties-values-api/#computationally-independent

The note in here gives an example of "1in" being computationally independent and "3em" not. It would be even better if this clarified whether units like "vw", which are relative to something, but not something controllable by the page, count as computationally independent.