The HTML event loop spec explicitly provides guidance for reentrancy in
the "perform a microtask checkpoint" algorithm, so we cannot VERIFY
for empty execution context before this early exit (as much as we'd
like to).
https://html.spec.whatwg.org/multipage/webappapis.html#perform-a-microtask-checkpoint
See the microtask-checkpoint-reentrancy-via-responsexml-script test for
an example of how this can happen from user scripts.
This fixes a browser crash as experienced on Wikipedia when encountering
the ≠ entity. As a side-effect, this also affects some tab-align and
-wrap tests.
