Codex ports a Python HTML parser to JavaScript
Willison supplied an existing parser and an independent compatibility suite, reviewed an API plan, then let Codex implement a JavaScript port through repeated tests and commits.
simonwillison.netWhat happened
Willison reports a dependency-free JavaScript library passing thousands of compatibility tests; the session summary also lists skipped cases.
Reported by Simon WillisonHow the agent did it
- Read the Python parser and compatibility tests and prepare an API specification.
- Implement a small working parser before expanding through the planned milestones.
- Run the compatibility suite repeatedly and commit progress, then add a browser playground.
Where a person stepped in
Willison reviewed the specification and smoke milestone, monitored commits and resumed the run after a usage limit.
A person approved each step before the agent continued.
More details
About this account
Codex worked through implementation milestones and test failures between a small number of follow-up instructions.
Firsthand technical write-up links code and tests. Compatibility is a reported outcome, not a guarantee of production suitability.
Credit: Simon Willison
Added 14 Oct 2026
Source published 15 Dec 2025
Source checked 14 Oct 2026
Tools and topics
Codex CLI · GPT-5.2 · html5lib-tests · Git · GitHub Actions
Code · Testing
Editorial completeness: 100/100. Measures documented fields, not agent performance or verified outcomes.
Prompt to try
Editorial adaptation, not the source author’s exact prompt. Replace bracketed inputs and review tool permissions first.
Port [library] to [language] in a separate directory. Inspect its license, API and independent compatibility suite. Write a plan and implement one smoke case first. Continue through the suite, preserving skipped and failing cases in the report. Stop before release.