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

Simon Willison used GPT-5.2 through Codex CLI to port Emil Stenström's JustHTML Python library to JavaScript in just 4 hours, creating a dependency-free HTML5 parser that passes 9,200 tests. The AI agent autonomously wrote 9,000 lines of code across 43 commits while Willison decorated for Christmas and watched a movie, demonstrating how coding agents can handle complex multi-hour tasks with minimal supervision.