Fix broken links to "layout viewport"

[tidoust]

The "layout viewport" term is now defined in CSS Viewport: https://drafts.csswg.org/css-viewport/#layout-viewport

This update fixes the link, leveraging ReSpec's cross-spec referencing capabilities to avoid hardcoding a URL that can evolve over time.

(Also using the opportunity to refresh the nearby link to "px")

[dandclark]

Suggested change

	, "css-viewport"
	, "cssom-view"

When this is css-viewport, the link currently resolves to https://www.w3.org/TR/css-viewport-1/#layout-viewport which doesn't actually have that definition. I think the issue is that the draft https://drafts.csswg.org/css-viewport/#layout-viewport defines the term so respec sees the linkage, but the published version https://www.w3.org/TR/css-viewport-1/ doesn't yet have it.

One option is to wait to land this PR until another version of css-viewport is published, at which point this link should be there. Another is to apply the suggestion to change it to cssom-view, which makes it resolve to published version of the spec this already links to (https://www.w3.org/TR/cssom-view-1/#layout-viewport), so we're at least not hardcoding the link anymore.

Or if there's a way to get respect to link to the draft version of css-viewport that'd work too, but I'm not sure if that can be done without hardcoding it.

I'm not sure which is better, maybe the editors of those specs have a recommendation? Or I'd be fine to just land a version of this now pointing to cssom-view-1 since the result is we still keep linking to the same spec for now, with a note to consider switching to cssom-view later if the published version of that spec ends up with a definition that's more applicable to what EditContext is doing with it.