Depends on D20630. See https://discourse.phabricator-community.org/t/bug-in-src-parser-html-phutildomnode-php/2882/. Currently, we parse <x>1<2</x> as an <x> tag containing a <2</x> tag.
A more expected interpretation of this is a single <x> tag containing the content 1<2.
When we encounter a <, always start a new tag, even if we're currently in a tag.