I Ported JustHTML from Python to JavaScript with Codex CLI and GPT-5.2 in 4.5h

Simon Willison hit a literal “YOLO” switch on an AI coding tool and, while trimming a Christmas tree and watching a movie, it cranked out a 9,000‑line JavaScript HTML parser that passes 9,200 tests. The repo is live: simonw/justjshtml. The crowd reaction? Equal parts confetti and courtroom.

The hype camp is loud. Even Simon jumps in saying this proves a new kind of project is “massively more feasible” when you’ve got rock‑solid test suites like html5lib-tests. One commenter flexes history trivia: before today’s AI, Firefox’s HTML5 parser was written in Java and then mechanically translated to C++—so automated porting isn’t brand new, just faster and flashier now.

But the skeptics are caffeinated. A sharp reply warns that “passing tests” isn’t the same as behaving exactly like real browsers, and points to speed‑first parsers that trade accuracy for performance. Others side‑eye the ethics: is an AI‑assisted port skating close to copying? One commenter brings up the “is it legal—and even if it is, is it ethical?” question, while another argues that if tests and specs are complete, the AI result should be “no better, and no worse” than a human team’s.

Meanwhile, the jokes write themselves: the “—dangerously‑bypass‑approvals—” flag is getting meme’d as the “hold my eggnog” button, and the 1.4M tokens eaten by the model? “AI drank all the Christmas cookies,” quips one wag. Tech miracle or test‑suite party trick? The thread can’t decide, and that’s the fun.