HTML can travel over the network to a browser either in HTML syntax or XML syntax, also known as XHTML.
HTML5 and HTML/XHTML
The HTML5 standard defines both these syntaxes. The choice of syntax is indicated by the MIME type (which is sent in the HTTP Content-Type header): the MIME type for HTML syntax is text/html, and the MIME type for XHTML syntax is application/xhtml+xml.
This example shows an HTML document and an XHTML document including the relevant HTTP headers:
HTML document
HTTP/1.1 200 OK
Content-Type: text/html
<!DOCTYPE html>
<html lang=en>
<head>
<meta charset=utf-8>
<title>HTML</title>
</head>
<body>
<p>I am a HTML document</p>
</body>
</html>
XHTML document
HTTP/1.1 200 OK
Content-Type: application/xhtml+xml
<html xml:lang="en" xmlns="https://www.w3.org/1999/xhtml">
<head>
<title>XHTML</title>
</head>
<body>
<p>I am a XHTML document</p>
</body>
</html>
MIME type versus DOCTYPE
Before HTML5, the two syntaxes were defined by two separate specifications: HTML 4.01 and XHTML 1.0. According to the XHTML1 standard, you could use XHTML by declaring a special DOCTYPE. However, no browsers have ever implemented this, and the HTML5 standard has reversed the decision. If your page is sent as text/html, you are not using XHTML.
Instead, the proper MIME type must be present in the Content-Type HTTP header. If you only put the MIME type into an HTML meta tag like <meta http-equiv=…>, it will be ignored and treated like text/html.
If you serve your pages as text/html and believe that you are writing XHTML, you may face several problems, as described in these articles:
- No to XHTML an excellent article from Spartanicus
- Beware of XHTML by David Hammond
- Sending XHTML as text/html Considered Harmful by Ian Hickson
- XHTML's Dirty Little Secret by Mark Pilgrim
- XHTML - What's the Point? by Henri Sivonen
- XHTML is not for Beginners by Lachlan Hunt
Support
Most modern browsers support XHTML, including Firefox, Chrome, Safari, Opera, and Internet Explorer (since IE 9). (Internet Explorer 8 and older browsers instead show a download dialog box for unknown file types when they see an XHTML document with the correct XHTML MIME type.)
Also be aware that many popular {{Glossary("JavaScript")}} libraries and developer tools have limited or no support for XHTML.
Differences from HTML
See Properly Using CSS and JavaScript in XHTML Documents for a partial list of differences between HTML and XHTML.