The XML syntax of HTML. See 13 The XML syntax. What used to be known as XHTML. Essentially it would mean creating a XMLDocument in the output window by passing the contents of the HTML window to an XML parser rather than to an HTML parser. All modern browsers support this, but jsbin, codepen etc don't.
You'd need to report on syntax errors that could stop the XML parser from completing the parse and only show the resulting document if the parse completed successfully.
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