[css-viewport] Investigate reusing HTML's tokenize the features argument

[annevk]

WebKit's parser for <meta name=viewport> is essentially https://html.spec.whatwg.org/#concept-window-open-features-tokenize with two additional branches (modulo #8757). We should investigate if they can be the same or at least standardize them in a similar fashion to encourage code reuse.

[ByteEater-pl]

Any chance of bringing back @viewport? It's a stylistic need, so that would be a proper solution, not markup. And it would be usable with any content styled by CSS, not just HTML. Who and why expressed unwillingness to implement it and does it still stand?