[css-properties-values-api] Registering a property should cause us to validate var() fallback #360
Description
In #321 (comment) the issue of var() fallback for typed variables is brought up. We should probably cause var() to be invalid-at-computed-value-time if its fallback doesn't match its registered grammar (regardless of whether or not the fallback is being used). Actually setting the custom property to a value that doesn't match its grammar does this already, so this makes the fallback work a little more consistently.