What exactly does it do?

• Fixes or removes non-well formed tags and attributes (e.g. adds alt attributes to images if missing)
• Converts the markup to HTML5 (if it is XHTML for example)
• Reformats the HTML (line breaks, indents)

Example

Input:
Hello <a href="https://htmlinstant.com/">world<a

will be corrected to:
Hello <a href="https://htmlinstant.com/">world</a>