title: What is a polyglot document?
body: <p>In <a href="https://stackoverflow.com/questions/1076897/html5-syntax-html-vs-xhtml/1077151#1077151">this answer</a> to <a href="https://stackoverflow.com/questions/1076897/html5-syntax-html-vs-xhtml">this question</a>, <a href="https://stackoverflow.com/users/132537/lachlan-hunt">Lachlan-Hunt</a> writes the following:</p>
  
  <blockquote>
    <p>With HTML5, you can choose to use HTML-only syntax, meaning that it is only compatible with being served and processed as text/html it is not well-formed XML. Or use XHTML-only syntax, meaning that is is well-formed XML, but uses XML features that are not compatible with HTML. Or, you can write a <strong>Polyglot document</strong>, which is conforming and compatible with both HTML and XHTML processing (In principle, this is conceptually similar to writing XHTML 1.0 that conforms with Appendix C guidelines).</p>
  </blockquote>
  
  <p>What is a Polyglot document?</p>