Use legacy name alias concept

[miketaylr]

https://drafts.csswg.org/css-cascade-4/#legacy-name-alias

When the old property’s syntax is identical to or a subset of the value space of the new property’s syntax, the two names are aliased with an operation on par with case-mapping: at parse time, the old property is converted into the new property. This conversion also applies in the CSSOM, both for string arguments and property accessors: requests for the old property name transparently transfer to the new property name instead.

Perhaps we can use this instead of the concept of simple property alias.

(cf @zcorpan's suggestion in w3c/csswg-drafts#4839)

[miketaylr]

Helpful pointer from @gsnedders, #154 (comment)