[css-variables] Applicable namespace rules for variable expansion

[SimonSapin]

This is pretty much the same issue as #757, so we may want a similar behavior.

/* a.css */
@namespace foo "http://www.w3.org/1999/xhtml";
:root { --a: foo }

/* b.css */
@namespace foo "http://www.w3.org/1999/xlink";
:root { --b: attr(var(--a)|href) }

/* c.css */
@namespace foo "";
a:before { content: var(--b) }

[tabatkins]

Yup, we do. So the foo namespace is resolved to the null namespace on use in this example.

[tabatkins]

Closing, as this is the behavior that falls out of the spec as currently written.