Its source code is written in ANSI C for maximum portability and compiled binary files are available for a variety of platforms. It is available under the W3C Software Notice and License (a permissive, BSD-style license). Up-to-date versions are currently available as source code cloned from its GitHub git version control repository, or in binary packages for multiple operating systems from its GitHub Releases repository.
Examples of fixes it can make to bad HTML:
Straighten mixed-up tags
Fix missing or mismatched end tags
Add missing items (some tags, quotes, ...)
Report proprietary HTML extensions
Change layout of markup to predefined style
Transform characters from some encodings into HTML entities