https://drafts.csswg.org/selectors/#type-nmsp

If a default namespace is declared, compound selectors without type selectors in them still only match elements in that default namespace.

https://drafts.csswg.org/css-nesting/#nest-selector defines a & selector (which is not a type selector).

Then, consider

@namespace url("http://example.com/foo");
@namespace svg url("http://www.w3.org/2000/svg");
svg|a {
  &:hover {}
}

&:hover has no type selector, so as per Selectors, it won't match a svg|a:hover. That's how Gecko, Blink and WebKit behave.

But I think it's supposed to match to be consistent with what @tabatkins is saying in #5684 and #9804

Possibly, Selectors should say something like

If a default namespace is declared, compound selectors without type selectors in them still only match elements in that default namespace.