Here is the list of unexpected test failures when running php bin/parserTests.php --html2wt. But, others need investigation and fixing. This are results with a few unmerged patches in gerrit.
Definition Lists: Nesting: Multi-level (Parsoid only) <pre> inside a link 2. Link fragments separated by <i> and <b> tags 3. Link fragments inside <i> and <b> [Before] HTML without raw HTML enabled ($wgRawHtml==false) Code coverage: D flag for conversion rule Code coverage: N flag for conversion rule Decoding of HTML entities in indicator names for IDs (unambiguous) (T104196) Filter syntax for language variants HTML markups with conversion syntax in attribs, nested in other conversion blocks Language converter markup with block content LST Sections: Newfangled approach Namespaced link must have a title (bad fragment version) Namespaced link must have a title Redirect to category page RTL (\u200f) and LTR (\u200e) markers around category tags should be stripped RTL (\u200f) and LTR (\u200e) markers should not be stripped if followed by a text node Serialize new placeholder space without spans T146305: Don't break image parsing if nested language converter markup is in the caption. T153135: Nested language converter markup in definition list (code coverage) T75121: Infer extension name from typeOf if data-mw is not present Whitespace scrubbing in SOL position should account for SOL-sensitive wikitext markup