HN Reader

JustHTML https://github.com/EmilStenstrom/justhtml is a neat new Python library - it implements a compliant HTML5 parser in ~3,000 lines of code that passes the full existing 9,200 test HTML5 conformance suite.

Emil Stenström wrote it with a variety of coding agent tools over the course of a couple of months. It's a really interesting case study in using coding agents to take on a very challenging project, taking advantage of their ability to iterate against existing tests.

I wrote a bit more about it here: https://simonwillison.net/2025/Dec/14/justhtml/

if it isnt too much to ask, since you are already insanely familiar with the html parser semantics, can you write a postgres extension that can parse html inside postgres? usecase: cleaning rss feed items while storing

Is it really too much to do a little more editing of the LLM output for the blog post? There's 17 numbered and titled section headings, all of which are linkable to with anchors, and which mostly have two sentences each.

I'm not seeing 100% pass rates.

    $ uv run run_tests.py --check-errors -v

    FAILED: 8337/9404 passed (88.6%), 13 skipped

It seems this the parser is creating errors even when none are expected:

    === INCOMING HTML ===
    <math><mi></mi></math>

    === EXPECTED ERRORS ===
    (none)

    === ACTUAL ERRORS ===
    (1,12): unexpected-null-character
    (1,1): expected-doctype-but-got-start-tag
    (1,11): invalid-codepoint

This "passes" because the output tree still matches the expected output, but it is clearly not correct.

The test suite also doesn't seem to be checking errors for large swaths of the html5 test suite even with --check-errors, so it's hard to say how many would pass if those were checked.

while it's mentioned in the post, it seems to me a bit burried:

isn't this more like a port of `html5ever` from rust to python using LLM, as opposed to creating something "new" based on the test suite alone?

if yes, wouldn't be the distinction rather important?