Getting Started: Transitioning to HTML5 - HTML5 Mastery: Semantics, Standards, and Styling - page 22

Is XHTML gone?
Short answer: no. It is now known as XHTML5. However, the HTML5 specification
states that XHTML must no longer be served with the MIME type “text/html,” as was
commonly done with XHTML 1. x . A major reason this came into practice was that
Internet Explorer would not parse a page served as XML and would instead attempt
to download the page to disk instead of displaying it. This was not the only reason,
however. XML is very strict in its syntax, and the smallest validation error would cause
the web page to break and become unusable, with the error visible for the entire world
to see. So, XHTML syntax was often used but still delivered as HTML as a precaution.
What's all this noise about MIME types?
Multipurpose Internet Mail Extensions (MIME) types, also known as the media type ,
tell a web page what kind of data it is receiving. Obviously, the web browser would
want to handle an image vs. a text document very differently, so it is important to have
the means to tell it what kind of data it is being sent. Since XHTML and HTML look
very similar, the web browser needs to be told which it is dealing with. If it is XHTML,
it needs to parse it using an XML parser that adheres to the XML specification; if it is