html5lib is a Python library that parses HTML documents according to the WHATWG HTML specification, the same standard used by modern web browsers. Developers use it when they need to reliably parse real-world HTML that may be malformed or non-standard, as it handles edge cases the way browsers do rather than failing on invalid markup. It's useful for web scraping, HTML processing, and any application that needs to work with HTML from untrusted or variable sources.