JustHTML: a Python HTML5 parser built with coding agents that passes html5lib tests
A developer spent his off-hours using Copilot agent mode with several models to build a dependency-free HTML5 parser, and wrote up every stage, including a full restart.
The problem
HTML5 parsing is notoriously complex. A large public conformance suite gave the agents a clear measure of progress, which made a mostly agent-written parser realistic.
How they did it
- Connect the html5lib-tests suite at the start and use its pass rate as the goal.
- Iterate with the agent, then restart from scratch by porting logic from html5ever.
- Profile, delete unused code found through coverage analysis, and harden the parser with fuzzing.
Results
- JustHTML is about 3,000 lines of Python with 8,500+ tests passing.
- It passes 100% of the html5lib test suite.
- It passed 3 million generated webpages without any crashes.
As reported by the source (Friendly Bit (Emil Stenström)); AgentGid did not measure these figures.
This suits a solo developer or small team that is comfortable in Python and already has a strong public test suite to aim at; the case is rated advanced, and it took a full restart plus profiling and fuzzing. Watch the credit budget: Copilot's Pro plan ($10/mo) includes 1,500 monthly AI Credits, and long agent-mode iterations draw on them. Zed (Free + $10/mo, open source) lets you use your own API keys or external agents.
The agent used here
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