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How I wrote JustHTML, a Python-based HTML5 parser, using coding agents
- simonw 11mo agoJustHTML https://github.com/EmilStenstrom/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/ https://simonwillison.net/2025/Dec/14/justhtml/
- EmilStenstrom 11mo agoThanks for sharing simon! Writing a parser is a really good job for a coding agent, because there's a clear right/wrong answer. In this case, the path there is the challenging part. The hours I've spent trying to convince agents to implement adoption agency well... :)
- msephton 11mo agoRSS on website is erroring. I'd like to follow!
- EmilStenstrom 11mo agoThanks! Now fixed.
- gabrielsroka 11mo ago> 3,000 loc I cloned the repo and ran `wc -l` on the src directory and got closer to 9,500. Am i missing something? Edit: maybe you meant just the parser
- HPsquared 11mo agoBetter to use something like `cloc` which excludes blank and comment lines.
- minusf 11mo agowhile 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?
- EmilStenstrom 11mo agoDepending on your perspective, you can take away any of the two points. The first iteration of the project created a library from scratch, from the tests all the way to 100% test coverage. So even without the second iteration, it's still possible to create something new. In an attempt to speed it up, I (with coding agent) rewrote it again based on html5ever's code structure. It's far from a clean port, because it's heavily optimized Rust code, that isn't possible to port to Python (Rust marcos). And it still depended on a lot of iteration and rerunning tests to get it anywhere. I'm not pushing any agenda here, you're free to take what you want from it!
- minusf 11mo agoThank you for the clarification, that was not entirely clear to me from the post. You also mention that the current "optimised" version is "good enough" for every-day use (I use `bs4` for working with html), was the first iteration also usable in that way? Did you look at `html5ever` because the LLM hit a wall trying to speed it up?
- EmilStenstrom 11mo agoIt was usable! Yeah, the handler based architecture that I had built on was very dependent on object lookups and method calls, and my hunch was that I had hit a wall trying to optimize the speed. I was slower than html5lib still, so decided to go with another "code architecture" (html5ever) that was closer to the metal. Worked out in getting me ~60% faster than html5lib. As for bs4, if you don't change the default, you get the stdlib html.parser, which doesn't implement html5. Only works for valid HTML.
- simonw 11mo ago
- vivzkestrel 11mo agoif 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
- EmilStenstrom 11mo agoThe license is MIT, so feel free to expand this any way you want! No need to write a new parser from scratch.
- furyofantares 11mo agoIs 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.
- EmilStenstrom 11mo agoHi! Yes, the headers were LLM generated and the text were not. I didn't want the blog post to go on for ages, so I just wrote a few lines under each heading. Any ideas how to make it better, while not being too long?
- furyofantares 11mo agoI'd start by deleting all the numbered section headings, and add either a transition word (then, so) or a transition sentence (why you went from step n to step n+1 or after how much time or whatnot).
- EmilStenstrom 11mo agoNew iteration up. I kept the headings because they make the text easier to scan, but made them more descriptive. Added some transition words. Slight improvement I think.
- deleted 11mo ago[deleted]
- Aloisius 11mo agoI'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.
- EmilStenstrom 11mo agoHi! The expected errors are not standardized enough for it to make sense to enable --check-errors by default. If you look at the readme, you'll see that the only thing they're checking is that the _numbers of errors_ are correct. That said, the example you are pulling our out does not match that either. I'll make sure to fix this bug and other like it! https://github.com/EmilStenstrom/justhtml/issues/20 https://github.com/EmilStenstrom/justhtml/issues/20
- Aloisius 11mo agorun_tests.py does not appear to be checking the number of errors or the errors themselves for the tokenizer, encoding or serializer tests from html5lib-tests - which represent the majority of tests. There's also something off about your benchmark comparison. If one runs pytest on html5lib, which uses html5lib-test plus its own unit tests and does check if errors match exactly, the pass rate appears to be much higher than 86%: $ uv run pytest -v 17500 passed, 15885 skipped, 683 xfailed, These numbers are inflated because html5lib-tests/tree-construction tests are run multiple times in different configurations. Many of the expected failures appear to be script tests similar to the ones JustHTML skips.
- EmilStenstrom 11mo agoExcellent feedback. I'll have a look at the running of html5lib tests again.
- JackSlateur 11mo agoWhat an ugly code (I read encoding.py)