For programmatic checking of modern HTML documents, use the API provided by the W3C HTML Checker. To do that, call https://validator.w3.org/nu/ or the URL for another instance of the W3C HTML Checker, and consult the following documentation:

- W3C HTML Checker GET interface
- W3C HTML Checker POST interface
- W3C HTML Checker parameters
- W3C HTML Checker out=json format
- W3C HTML Checker out=xml format
- W3C HTML Checker out=gnu format