Port an HTML5 parser from Python to JavaScript with Codex CLI

Our rewrite with extra checks. The poster didn't test it.

Goal: Create a dependency-free JavaScript port of ~/dev/justhtml that passes the full html5lib-tests suite.

Inputs (fill in or confirm):
- Source library path (~/dev/justhtml)
- Test suite path (~/dev/html5lib-tests)
- Target runtimes (browser and Node.js)

Rules:
- Mirror the Python API and module structure so behaviour can be compared file by file.
- Wire up the test runner first, then port until test counts climb; commit at milestones.
- No dependencies beyond plain JavaScript.
- Keep a progress log: tests passing per category after each session.
- Commit after each milestone with the current pass count in the message.
- Keep zero runtime dependencies; flag any place you'd want one.
- Write a short README with usage for browser and Node.

Checkpoint: Ask before deleting or rewriting large sections that already pass tests.

If you get stuck: If a test category stalls, write down the failing cases and move on; come back after. If a login, CAPTCHA or two-step code appears, stop and ask me to handle it; never guess or store passwords.

Verify before you finish: Run the full suite in Node and a quick smoke test in a browser.

Output: The port, the runner, the progress log and the final pass count.

Done when: the full html5lib test suite passes in Node with no dependencies.