What exactly does it do?

Fixes malformed HTML (unclosed tags, invalid nesting)
Reduces the markup to: <html lang>, <head>, <meta charset name content>, <title>, <body>, <p>, <blockquote cite>, <hr>, <figure>, <figcaption>, <a href title target>, <strong>, <em>, <b>, <i>, <s>, <u>, <br>, <h1>, <h2>, <h3>, <h4>, <h5>, <h6>, <img src alt width height loading>, <picture>, <source srcset sizes media type src>, <video src width height poster controls preload>, <audio src controls preload>, <table>, <caption>, <thead>, <tbody>, <tfoot>, <tr>, <th colspan rowspan>, <td colspan rowspan>, <col span>, <colgroup span>, <code>, <pre>, <ul>, <ol start type reversed>, <li>, <dl>, <dt>, <dd>, <abbr title>, <cite>, <dfn>, <kbd>, <samp>, <var>, <mark>, <small>, <q>, <wbr>, <del datetime cite>, <ins datetime cite>, <sub>, <sup>, <time datetime>
Replaces: <strike> to <del>, <tt> to <code>, <acronym> to <abbr>, <dir> to <ul>, <listing> to <pre>, <xmp> to <pre>, <plaintext> to <pre>
Reformats the HTML (line breaks, indents)

Clean up HTML docs & convert Word to clean HTML