[csswg-drafts] [css-variables] Applicable namespace rules for variable expansion

SimonSapin has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-variables] Applicable namespace rules for variable expansion ==
This is pretty much the same issue as https://github.com/w3c/csswg-drafts/issues/757, so we may want a similar behavior.

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

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/1163 using your GitHub account