XHTML is a reformulation of HTML 4 as an XML 1

XHTML

XHTML is a reformulation of HTML 4 as an XML 1.0 application. The stricter nature of XML requires following XML rules exactly when creating HTML documents.

The most important rules for creating valid XHTML:

Include the correct DOCTYPE declaration at the beginning of the file.
Add the attribute xmlns="http://www.w3.org/1999/xhtml" to the <html> tag.
Close all tags: Omitting </p> is not valid anymore.
Change all tags to lowercase: <P> becomes <p>.
Correctly specify empty elements: <hr> becomes <hr />.
Quote all attribute values: <p align="right">.
Always add attribute values: <hr noshade="noshade">.
Always use & for & in attributes: <a href="?a=1&b=2">.

Document Type	Namespace Description
Strict	<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"> The more rigorous, more XML-style version of XHTML
Transitional	<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"> The more forgiving, more backwards-compatible version of XHTML
Frameset	<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"> DTD for frames

DTD: indicates in which XHTML 1.0 DTD the tag is allowed. S=Strict, T=Transitional, and F=Frameset

Start tag	Purpose	DTD

Basic Tags
<!DOCTYPE>	Defines the document type	STF
<html>	Defines an html document	STF
<body>	Defines the body element	STF
<h1> to <h6>	Defines header 1 to header 6	STF
<p>	Defines a paragraph	STF
<br/>	Inserts a single line break	STF
<hr/>	Defines a horizontal rule	STF
<!--...-->	Defines a comment	STF

Fonts
<b>	Defines bold text	STF
<font>	Deprecated. Defines text font, size, and color	TF
<i>	Defines italic text	STF
<em>	Defines emphasized text	STF
<big>	Defines big text	STF
<strong>	Defines strong text	STF
<small>	Defines small text	STF
<sup>	Defines superscripted text	STF
<sub>	Defines subscripted text	STF
<bdo>	Defines the direction of text display	STF
<u>	Deprecated. Defines underlined text	TF

Text Formatting
<pre>	Defines preformatted text	STF
<code	Defines computer code text	STF
<tt>	Defines teletype text	STF
<kbd>	Defines keyboard text	STF
<var>	Defines a variable	STF
<dfn>	Defines a definition term	STF
<samp>	Defines sample computer code	STF
<xmp>	Deprecated. Defines preformatted text

Blocks
<acronym>	Defines an acronym	STF
<abbr>	Defines an abbreviation	STF
<address>	Defines an address element	STF
<blockquote	Defines a long quotation	STF
<center>	Deprecated. Defines centered text	TF
<q>	Defines a short quotation	STF
<cite>	Defines a citation	STF
<ins>	Defines inserted text	STF
<del>	Defines deleted text	STF
<s>	Deprecated. Defines strikethrough text	TF
<strike>	Deprecated. Defines strikethrough text	TF

Links
<a>	Defines an anchor	STF
<link>	Defines a resource reference	STF

Frames
<frame>	Defines a sub window (a frame)	F
<frameset>	Defines a set of frames	F
<noframes>	Defines a noframe section	TF
<iframe>	Defines an inline sub window (frame)	TF

Forms
<form>	Defines a form	STF
<input>	Defines an input field	STF
<textarea	Defines a text area	STF
<button>	Defines a push button	STF
<select>	Defines a selectable list	STF
<optgroup>	Defines an option group	STF
<option>	Defines an item in a list box	STF
<label>	Defines a label for a form control	STF
<fieldset>	Defines a fieldset	STF
<legend	Defines a title in a fieldset	STF
<isindex>	Deprecated. Defines a single-line input field	TF

Lists
<ul>	Defines an unordered list	STF
<ol>	Defines an ordered list	STF
<li>	Defines a list item	STF
<dir>	Deprecated. Defines a directory list	TF
<dl>	Defines a definition list	STF
<dt>	Defines a definition term	STF
<dd>	Defines a definition description	STF
<menu>	Deprecated. Defines a menu list	TF

Images
<img src="url" />	Defines an image	STF
<map>	Defines an image map	STF
<area>	Defines an area inside an image map	STF

Tables
<table>	Defines a table	STF
<caption>	Defines a table caption	STF
<th>	Defines a table header	STF
<tr>	Defines a table row	STF
<td>	Defines a table cell	STF
<thead>	Defines a table header	STF
<tbody	Defines a table body	STF
<tfoot>	Defines a table footer	STF
<col>	Defines attributes for table columns	STF
<colgroup	Defines groups of table columns	STF

Styles
<style>	Defines a style definition	STF
<div>	Defines a section in a document	STF
<span>	Defines a section in a document	STF

Meta Information
<head>	Defines information about the document	STF
<title>	Defines the document title	STF
<meta>	Defines meta information	STF
<base>	Defines a base URL for all the links in a page	STF
<basefont>	Deprecated. Defines a base font	TF

Scripting
<script>	Defines a script	STF
<noscript>	Defines a noscript section	STF
<applet>	Deprecated. Defines an applet	TF
<object>	Defines an embedded object	STF
<param>	Defines a parameter for an object	STF