[css-anchor-position-1] Depluralize anchors-* in 'position-visibility' and only check default anchor #10201

[fantasai]

See #10201. While making the edits I noticed that the renaming would make anchors-visible inconsistent with anchor-valid, so opening a PR to check with the WG whether we want to:

• Rename both.
• Rename only one. :(
• Rename both and add legacy aliases.
• Something else.

[tuankiet65]

I'll assume if renamed, anchor-visible would only care about the default anchor like anchor-valid, right? Then I support 1) renaming both and 2) change their definition to only care about the default anchor.

I don't support adding aliases because it doesn't make sense to keep the plural form when we only care about one anchor (the default one). Also I don't want to carry historical baggage.

[emilio]

Yeah, anchors-visible caring only about the default anchor but being plural is indeed quite weird. I'd support renaming to anchor-visible... But a parse time alias might be cheap enough and worth it?