This sort of white-list always scares me because it's not future proof.

Also note that `steps` and `cubic-bezier` take arguments so checking for them in a list is not going to work. We need to iterate over the list of possible timing functions and do a `str-index` on the argument.

Personally, I'd rather not white list this at all. Like times, I think the first string should be considered the property and the second string considered a timing function.