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HTML Tidy is a console application whose purpose is to fix invalid HTML, detect potential web accessibility errors, and improve the layout and indent style of the resulting markup. It is also a cross-platform library that can be used by programmers in computer applications to add HTML Tidy's capabilities to them.
It was first developed by Dave Raggett  of World Wide Web Consortium (W3C), then released as a SourceForge project in 2003 and managed by various maintainers.
In 2012 the project was moved to GitHub and maintained by Michael Smith, also of W3C, where critical HTML5 support was added.
In 2015 the HTML Tidy Advocacy Community Group (HTACG) was formed to take over management and development of HTML Tidy as a W3C Community Group.
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 only as source code, cloned from its GitHub git version control 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