All of the prompts I used are in the article. The two most relevant to testing were:

  We are going to create a JavaScript port of ~/dev/justhtml - an HTML parsing library that passes the full ~/dev/html5lib-tests test suite. [...]

And later:

  Configure GitHub Actions test.yml to run that on every commit, then commit and push

Good coding models don't need much of a push to get heavily into automated testing.

I used Codex for a few reasons:

1. Claude was down on Sunday when I kicked off tbis project

2. Claude Code is my daily driver and I didn't want to burn through my token allowance on an experiment

3. I wanted to see how well the new GPT-5.2 could handle a long running project