Created attachment 277423 [details]
STEPS TO REPRODUCE: Load attached testcase from a .xhtml filename.
EXPECTED RESULTS: The two alerts should show the same text.
ACTUAL RESULTS: Two alerts showing different text.
ADDITIONAL INFORMATION: Looks like the innerHTML setter on script/style sets the textContent even in XML, but the getter serializes as XML and ends up escaping things.
See also https://github.com/w3c/DOM-Parsing/issues/6 and https://bugs.chromium.org/p/chromium/issues/detail?id=605973 for the Blink equivalent of this bug.