Real Agent Cases

Port an HTML5 parser from Python to JavaScript with Codex CLI

Source date:
Last verified:
Method:
How we verify

Evidence: Screenshots or logsThe source shows screenshots, a transcript or the agent's output.

Outcome

Codex CLI (GPT-5.2) ran for hours largely unattended and produced about 9,000 lines of dependency-free JavaScript passing 9,200 html5lib tests, in about 4.5 hours start to finish.

Poster says:Passes 9,200 html5lib tests

This is the source's own number. We haven't measured it.

Agent

OpenAI Codex CLI

Prompt Original prompt

Exact prompt as published in the source.

We are going to create a JavaScript port of ~/dev/justhtml - an HTML parsing library that passes the full ~/dev/html5lib-tests test suite. It is going to have a similar API to the Python library but in JavaScript. It will have no dependencies other than raw JavaScript, hence it will work great in the browser and node.js and other environments. Start by reading ~/dev/justhtml and designing the user-facing API for the new library - create a spec.md containing your plan.

Source

https://simonwillison.net/2025/Dec/15/porting-justhtml/

By Simon Willison · Published Dec 15, 2025 · Added Oct 15, 2026 · Last verified Oct 15, 2026

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