perf(is-hidden-with-css): Cache result (#1846)
perf(is-hidden-with-css): Cache result (#1846)

* perf(is-hidden-with-css): Cache result

Fixes https://github.com/dequelabs/axe-core/issues/1717.

* refactor(is-hidden-with-css): Use virtual node

* fix: Handle missing virtual node

Seems that `body` doesn't have a virtual node (at least in tests).

dequelabs/axe-core,dequelabs/axe-core,dequelabs/axe-core,dequelabs/axe-core