Focusable: Fix handling of `visibility: collapse`

Focusable: Fix handling of `visibility: collapse`

"collapse" is similar to "hidden", with a slight difference in the case
of tr/tbody/td/colgroup elements.
See https://www.w3.org/TR/CSS22/visufx.html#visibility
See https://www.w3.org/TR/CSS22/tables.html#dynamic-effects
See https://developer.mozilla.org/en-US/docs/Web/CSS/visibility#Table_example

"visibility: collapse" elements are always not focusable, though.

Commit d3025968f34 introduced a regression by testing with `!== "hidden"`
instead of `=== "visible"`.

Closes gh-1843