The html5lib_*.html files in this directory wrap the html5lib-tests
tree-construction .dat files in resources/. Each wrapper covers one of
the three input modes: html5lib_url.html (Blob URL),
html5lib_write.html (document.write), and
html5lib_write_single.html (one-character-at-a-time document.write).
Per-test dispatch routes #document-fragment entries through innerHTML
regardless of the wrapper's mode.

The .dat files and the wrappers are regenerated by

  wpt update-built --include html5lib

which reads ../tools/html5lib_tests_revision and exports tree-construction
.dat files at that SHA from a local html5lib-tests clone (defaulting to
../../../html5lib-tests). To bump the upstream revision, edit the
revision file and re-run.