XHTML5

XHTML5 is the XML serialization of HTML5, meaning it is extended from XML and HTML5. XHTML5 must be served with an XML MIME type:. XHTML5 requires strict well-formed syntax.

Error handling
An HTML5 (text/html) browser will be flexible in handling incorrect syntax. HTML5 is designed so that old browsers can safely ignore new HTML5 constructs. In contrast to HTML 4.01, the HTML5 specification gives detailed rules for lexing and parsing, with the intent that different compliant browsers will produce the same result in the case of incorrect syntax. Although HTML5 now defines a consistent behavior for "tag soup" documents, those documents are not regarded as conforming to the HTML5 standard.