Post

Simon Willison
@simonwillison.net

I ported a Python library implementing a full HTML5 parser to JavaScript using GPT-5.2 and Codex CLI in 4.5 hours, and decorated for Christmas and watched Knives Out while I was doing it simonwillison.net/2025/Dec/15/...

simonwillison.netI ported JustHTML from Python to JavaScript with Codex CLI and GPT-5.2 in 4.5 hoursI wrote about JustHTML yesterday—Emil Stenström’s project to build a new standards compliant HTML5 parser in pure Python code using coding agents running against the comprehensive html5lib-tests testi...
Simon Willison
@simonwillison.net

I'm sharing this because it's quite a stark demonstration of what's possible using the latest frontier LLMs at the tail end of 2025 Is it a GOOD idea to churn out a 9,000 line direct port while watching Daniel Craig chew on the scenery? That's a whole other set of questions!

isaacs
@isaacs.bsky.social

Impressive! I'm curious, the license says it's copyright you. Is that accurate? Who actually owns the IP, if an AI agent wrote all the code? IANAL and even less a court, so I don't really know, but it's also interesting to me, and it's likely central to some of the questions at the end of the post

Simon Willison
@simonwillison.net

That's one of the questions I should have included in the post, I'll add that now I don't know for certain. Most of my AI-assisted code involves WAY more of my own interactions - in this case the majority of the input was the code written by Emil for JustHTML itself

0 reposts6 likes