I wrote JustHTML using coding agents

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/

Thanks 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... :)

RSS on website is erroring. I'd like to follow!