Case Studygpt 5 2html5 parsercodex cli
Developer Ports JustHTML Using GPT-5.2 To JavaScript
8.0
On 15 December 2025, developer Simon Willison ported Emil Stenström’s JustHTML to JavaScript by driving GPT-5.2 via Codex CLI, producing simonw/justjshtml that passes roughly 9,200 html5lib-tests. The end-to-end work took about four hours, consumed over 1.46 million input tokens, and produced a dependency-free parser with CI, CSS selectors, and a browser playground.
Key Points
- 1Ports JustHTML to JavaScript passing 9,200 html5lib-tests using GPT-5.2 and Codex CLI
- 2Demonstrates LLMs' sustained multi-hour code reasoning across 1.46M input tokens and 625K outputs
- 3Enables practitioners to accelerate library ports, CI integration, and browser/node no-deps deployments
Scoring Rationale
Demonstrates practical, reproducible LLM-assisted porting with high credibility, but limited to a specific library and tooling setup.
Sources
Public references used for this report.
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