I ported JustHTML from Python to JavaScript with Codex CLI and GPT-5.2 in 4.5 hours

Thu, 18 Dec 2025 07:37:00 -0400

Simon Willison, Dec 18, 2025

A couple of weeks ago Emil Stenström wrote How I wrote JustHTML using coding agents. I read it at the time - I thought maybe I had posted it here, but I guess I hadn't (it's one of those niche posts that really interesting to me but maybe less interesting to the broader e-learning readership). It describes using AI to write a fully compliant HTML5 parser in Python (not trivial, because there are so many ways to write HTML incorrectly, and a parser can't choke on them). It was significant to me because it suggests that testing, rather than reading lines of code, will be how we validate software in the future. Anyhow, in this article Simon Willison describes porting the software from Python to Javascript - "It took two initial prompts and a few tiny follow-ups... Time elapsed from project idea to finished library: about 4 hours, during which I also bought and decorated a Christmas tree with family and watched the latest Knives Out movie." So is that what software development is now? "Is it responsible and appropriate to churn out a direct port of a library like this in a few hours while watching a movie? What would it take for code built like this to be trusted in production?" Here's the playground for the new software. Works perfectly.