I Ported JustHTML from Python to JavaScript with Codex CLI and GPT-5.2 in 4.5hrs (simonwillison.net)

In a remarkable demonstration of AI capabilities, a developer ported JustHTML, an HTML5 parser originally written in Python, to JavaScript using Codex CLI and GPT-5.2 in just 4.5 hours. The resulting library, simonw/justjshtml, is a dependency-free HTML5 parser that successfully passes 9,200 tests from the html5lib-tests suite and mirrors the API design of the original JustHTML. The process involved minimal prompts and allowed the developer to multitask, showcasing the efficiency and potential of using advanced coding agents for software development. This achievement is significant for the AI/ML community as it illustrates the effective application of large language models (LLMs) for complex coding tasks with minimal human intervention. The success was attributed to the availability of comprehensive testing suites, underscoring the importance of solid testing frameworks in enabling such autonomous coding processes. This also raises various ethical and legal questions regarding copyright and the impact of AI-generated code on the open-source ecosystem, prompting discussions on responsible AI usage in software development.