Porting JustHTML HTML5 Parser from Python to JavaScript Using Codex CLI and GPT-5.2
By
pbowyer
Master baker tier. Every paragraph earns its place on the tray.
Summary
The author describes successfully porting JustHTML, a standards-compliant HTML5 parser originally written in Python, to JavaScript using Codex CLI and GPT-5.2 in just 4.5 hours. The resulting JavaScript library (simonw/justjshtml) is dependency-free, passes 9,200 tests from the html5lib-tests suite, and imitates the original Python API. The article details the technical process, challenges encountered, and impressive results achieved with AI-assisted coding tools.
Key quotes
· 5 pulledI decided to try porting JustHTML from Python to JavaScript with the least amount of effort possible, using Codex CLI and GPT-5.2. It worked beyond my expectations.
I built simonw/justjshtml, a dependency-free HTML5 parsing library in JavaScript which passes 9,200 tests from the html5lib-tests suite and imitates the API
The entire process took me 4.5 hours, from starting with the Python code to having a working JavaScript library that passes 9,200 tests.
What surprised me most was how well GPT-5.2 understood the structure and intent of the Python code, making intelligent decisions about how to translate Python idioms to JavaScript equivalents.
This experience has convinced me that AI-assisted programming tools have reached a point where they can dramatically accelerate certain types of software development tasks.
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