Consider something like this: <pre>Some text<br />More text</pre> When we run that through our XHTML serializer, we increment mPreLevel at the <pre>, increment it again at the <br> start, do NOT decrement it at the <br> end because it's self-closing, and then end up with a nonzero mPreLevel forever after. And our serializer tests don't catch this, because they run with a display:none iframe, where we never end up touching mPreLevel at all, because it's style-dependent.

The test change makes sure the test actually tests this codepath. The resulting test changes are all due to the test now recognizing <pre> as preformatted.

Pushed by bzbarsky@mozilla.com: https://hg.mozilla.org/integration/mozilla-inbound/rev/1fdcea1595a4 Fix the XHTML serializer's handling of self-closing tags inside <pre> to correctly keep track of our mPreLevel. r=ehsan