add CSS Extensions meant for renaming

[Nick-Levinson]

I'm thinking of rewriting my long stylesheets to make them more concise using CSS Extensions. The term "CSS Extensions" has two meanings, for both proprietary extensions to CSS, such as -moz-* properties, and those described in https://drafts.csswg.org/css-extensions/, which is partly for letting a "repeatedly-used value . . . be given a short, memorable name instead". I want to use the latter kind of CSS Extensions if most of the modern browsers will use them. Please add reports of support for the latter meaning of CSS Extensions.