Cars & Trucks

and

.) The specification is expected to add WAI-ARIA support in a future draft. There is currently no indication as to whether HTML 5 will support RDFa, or be limited just to [[microformats]]. == Valid XHTML documents == An XHTML document that conforms to an XHTML specification is said to be ''valid''. Validity assures consistency in document code, which in turn eases processing, but does not necessarily ensure consistent rendering by browsers. A document can be checked for validity with the [[W3C Markup Validation Service]]. In practice, many web development programs such as [[Dreamweaver]] provide code validation based on the [[W3C]] standards. === DOCTYPEs === In order to validate an XHTML document, a [[Document Type Declaration]], or ''DOCTYPE'', may be used. A DOCTYPE declares to the browser which [[Document Type Definition]] (DTD) the document conforms to. A Document Type Declaration should be placed before the [[root element]]. The [[system identifier]] part of the DOCTYPE, which in these examples is the [[Uniform Resource Locator|URL]] that begins with ''http://'', need only point to a copy of the DTD to use if the validator cannot locate one based on the [[public identifier]] (the other quoted string). It does not need to be the specific URL that is in these examples; in fact, authors are encouraged to use local copies of the DTD files when possible. The public identifier, however, must be character-for-character the same as in the examples. These are the most common XHTML Document Type Declarations: ;XHTML 1.0 Strict :

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

;XHTML 1.0 Transitional :

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

;XHTML 1.0 Frameset :

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">

;XHTML 1.1 :

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">

;HTML 5 HTML5 does not require a doctype, and HTML 5 validation is not DTD-based. ;XHTML 2.0 XHTML 2.0, [[As of April 2008]], is in a draft phase. If an XHTML 2.0 Recommendation is published with the same document type declaration as in the current Working Draft, the declaration will appear as: :

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 2.0//EN"
"http://www.w3.org/MarkUp/DTD/xhtml2.dtd">

A placeholder DTD schema exists at the corresponding URI, though it currently only includes the character reference entities from previous recommendations. XHTML 2 contemplates both a version attribute and an xsi:schemalocation attribute on the root HTML element that could possibly serve as a substitute for any DOCTYPE declaration. ==== XML namespaces and schemas ==== In addition to the DOCTYPE, all XHTML elements must be in the appropriate [[XML namespace]] for the version being used. This is usually done by declaring a default namespace on the root element using xmlns="namespace" as in the example below. For XHTML 1.0, XHTML 1.1 and HTML5, this is :<html xmlns="http://www.w3.org/1999/xhtml"> XHTML 2.0 requires both a namespace and an [[W3C XML Schema|XML Schema]] instance declaration. These might be declared as :

<html xmlns="http://www.w3.org/2002/06/xhtml2/"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.w3.org/2002/06/xhtml2/ http://www.w3.org/MarkUp/SCHEMA/xhtml2.xsd">

This example for XHTML 2.0 also demonstrates the use of multiple namespaces within a document. The first xmlns default namespace declaration indicates that elements and attributes whose names have no XML namespace prefix fall within the XHTML 2.0 namespace. The second namespace prefix declaration xmlns:xsi indicates that any elements or attributes prefixed with the xsi: refer to the XMLSchema-Instance namespace. Through this namespace mechanism XML documents allow the use of a mixture of elements and attributes taken from various XML vocabularies while avoiding the potential for clashes of naming between items from independently developed vocabularies. Similar to the case of DOCTYPE above, the actual URL to the [[W3C XML Schema|XML Schema]] file can be changed, as long as the [[Universal Resource Identifier]] (URI) before it (which indicates the XHTML 2.0 namespace) remains the same. The namespace URI is intended to be a persistent and universally unique identifier for the particular version of the specification. If treated as a URL, the actual content located at the site is of no significance. ==== XML Declaration ==== A [[character encoding]] may be specified at the beginning of an XHTML document in the XML declaration when the document is served using the application/xhtml+xml MIME type. (If an XML document lacks encoding specification, an XML parser assumes that the encoding is [[UTF-8]] or [[UTF-16]], unless the encoding has already been determined by a higher protocol.) For example: : The declaration may be optionally omitted because it declares as its encoding the default encoding. However, if the document instead makes use of XML 1.1 or another character encoding, a declaration is necessary. [[Internet Explorer]] prior to version 7 enters [[quirks mode]] if it encounters an XML declaration in a document served as text/html. === Common errors === Some of the most common errors in the usage of XHTML are: * Failing to realize that documents won’t be treated as XHTML unless they are served with an appropriate XML [[Internet media type|MIME type]] * Not closing empty elements (elements without closing tags in HTML4) ** Incorrect:

** Correct:

Note that any of these are acceptable in XHTML: <br></br>, <br/> and <br />. Older HTML-only browsers interpreting it as HTML will generally accept <br> and <br />. * Not closing non-empty elements ** Incorrect: This is a paragraph.

This is another paragraph. ** Correct:

This is a paragraph.

This is another paragraph.

* Improperly nesting elements (Note that this would also be invalid in [[HTML]]) ** Incorrect: This is some text. ** Correct: This is some text. * Not putting quotation marks around attribute values ** Incorrect: ** Correct: ** Correct: * Using the ampersand character outside of entities ** Incorrect: Cars & Trucks ** Correct: Cars & Trucks * Using the ampersand outside of entities in [[Uniform Resource Locator|URL]]s (Note that this would also be invalid in [[HTML]]) ** Incorrect: News ** Correct: News * Failing to recognize that XHTML elements and attributes are case sensitive ** Incorrect: The Best Page Ever ** Correct: The Best Page Ever * Using attribute minimization ** Incorrect: READ-ONLY ** Correct: READ-ONLY * Mis-using CDATA, script-comments and xml-comments when embedding scripts and stylesheets. ** This problem can be avoided altogether by putting all script and stylesheet information into separate files and referring to them as follows in the XHTML head element.

 <link rel="stylesheet" href="/style/screen.css" type="text/css" />
 <script type="text/javascript" src="/script/site.js"></script>

::Note: The format <script …></script>, rather than the more concise <script … />, is required for HTML compatibility when served as MIME type text/html. ** If an author chooses to include script or style data inline within an XHTML document, different approaches are recommended depending whether the author intends to serve the page as application/xhtml+xml and target only fully conformant browsers, or serve the page as text/html and try to obtain usability in Internet Explorer 6 and other non-conformant browsers. == Backward compatibility == XHTML 1.x documents are mostly backward compatible with HTML 4 user agents when the appropriate guidelines are followed. XHTML 1.1 is essentially compatible, although the elements for [[ruby character|ruby annotiation]] are not part of the HTML 4 specification and thus generally ignored by HTML 4 browsers. Later XHTML 1.x modules such as those for the role attribute, [[RDFa]] and [[WAI-ARIA]] degrade gracefully in a similar manner. HTML 5 and XHTML 2 are significantly less compatible, although this can be mitigated to some degree through the use of scripting. (This can be simple one-liners, such as the use of “document.createElement()” to register a new HTML element within Internet Explorer, or complete JavaScript frameworks, such as the [[FormFaces]] implementation of [[XForms]].) === Examples === The followings are examples of XHTML 1.0 Strict. Both of them have the same visual output. The former one follows the [http://www.w3.org/TR/xhtml1/#guidelines HTML Compatibility Guidelines] in Appendix C of the XHTML 1.0 Specification while the latter one breaks backward compatibility but provides cleaner codes. {| class="wikitable" |+ style="text-align: left;" | Media type recommendation for the examples: |----- ! width="250" height="28" | Media type ! width="100" height="28" | Example 1 ! width="100" height="28" | Example 2 |----- ! width="250" height="25" | application/xhtml+xml | width="100" height="25" | SHOULD || width="100" height="25" | SHOULD |----- ! width="250" height="25" | application/xml | width="100" height="25" | MAY || width="100" height="25" | MAY |----- ! width="250" height="25" | text/xml | width="100" height="25" | MAY || width="100" height="25" | MAY |----- ! width="250" height="25" | text/html | width="100" height="25" | MAY | width="100" height="25" | SHOULD NOT |} Example 1. XHTML 1.0 Example

This is an example of an XHTML 1.0 Strict document.

Example 2. XHTML 1.0 Example

This is an example of an XHTML 1.0 Strict document.