Consider this testcase: <button style="visibility:hidden; opacity: 0.99"> <span style="visibility: visible">First</span> </button> <button style="visibility:hidden; opacity: 1"> <span style="visibility: visible ">Second</span> </button> "Second" shows up but "First" does not. This seems pretty odd. You can get the same effect by having both buttons positioned and one with non-auto z-index (so it's a stacking context). In general, this check seems bogus to me at first glance.

Looks like it's been there all the way since bug 317375.