`@font-face` rules cannot contain nested selectors. Nor can they be
nested under a selector. Normally this would be a syntax error by the
author of the styles. But in some rare cases, such as importing styles
from a library, and applying `:host ::ng-deep` to the imported styles,
we can end up with broken css if the imported styles happen to contain
`@font-face` rules.
This commit works around this problem by sanitizing such cases (erasing
any scoping selectors) during emulated ShadowDOM encapsulation style
processing.
Fixes#41751
PR Close#41815
da6ed1562e