xhtml Getting started with xhtml Installation or Setup
Example
XHTML Relation to HTML
XHTML is simply the serialized version of HTML. XHTML was originally intended to clean up HTML markup to better improve standards support. Unfortunately the W3C's work on XHTML 2.0 was not intuitive and detracted from where the industry as a whole was headed with HTML5. XHTML5 is not a standard though you can use XHTML5 (XML serialized HTML5) with minor modifications. Examples are provided below to ease the adoption of XHTML5 and clarify ambiguities in the HTML5 standard where XML is not taken in to consideration.
XHTML Benefits
Because XHTML uses a browser's XML parser it is significantly (though not absolutely) less prone to common programming errors. If a developer loads an XHTML application with malformed XML most browser rendering engines will render up to the malformed XML while the Gecko rendering engine (used in Firefox) will show a yellow page. In all cases an error message will be displayed with the line and column numbers of where the first encountered XML parsing error was encountered. Many developers have made simple mistakes such as missing a quote from an HTML attribute leaving them spending days trying to determine why one or two browsers are not rendering the HTML page as expected, using XHTML can dramatically decrease development time over HTML. XHTML may have a positive reinforcement on learning to code valid HTML for people learning web development as it does not allow the XHTML application to render in full with malformed code and by giving an explicit error message that new developers can search for online for working solutions. Since XHTML is a subset of XML is therefore has a very high compatibility with software that works with XML in a very wide array of industrial, commercial and residential uses. Lastly due to it's strict requirements working XHTML code is automatically compatible with HTML and XML (subjective to absence multiple same id attribute values) while HTML is not inherently compatible with other bodies of HTML and XML as it is not immediately apparent that the HTML code may be malformed.
XHTML Drawbacks
Due to the much stricter XML parser rules XHTML is not as easy, initially, for less seasoned developers. XHTML is not taken in to consideration often by various groups, contains ambiguity in regards to poorly written parts of the HTML5 standard and there is no explicit validator that declares support for XHTML5. XHTML is less compatible with "lazier" parts of JavaScript such as innerHTML which does not properly serialize new parts of the DOM however this is more of a benefit to dedicated developers learning XHTML as it requires stricter and more interchangeable code.
Using XHTML
XHTML is the combination of HTML and the use of the XML parser which is a much stricter version of the HTML parser; all modern browsers have HTML parsers for HTML and XML parsers for XML (and subsequently XHTML). XHTML does not require an installation of software (beyond using any modern browser) however an (X)HTML application is only XHTML when it is correctly served to the browser with a header sent by the server declaring the application/xhtml+xml mime type; you can verify if your XHTML application was actually served as this by viewing the media type/mime/type in the network requests panel of the web developer tools in your browser. A page served as text/html is not XHTML and will instead be parsed by browser's HTML parser.
Loaded from a Server
When loading an XHTML parser from the server a header must be set, using a meta element serves no useful purpose. In example in combination with PHP:
<?php
if (isset($_SERVER['HTTP_ACCEPT']) && stristr($_SERVER['HTTP_ACCEPT'],'application/xhtml+xml'))
{
// Client header isset` and explicitly declares XHTML parser support.
header('Content-Type: application/xhtml+xml; charset=UTF-8');
echo '<?xml version="1.0" encoding="UTF-8"?>'."\n";
echo '<!DOCTYPE html>'."\n";
echo '<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">'."\n";
}
else
{
//Some browsers do not declare support, IE9 shamelessly uses `*/*` in example.
echo '<!DOCTYPE html>'."\n";
echo '<html>'."\n";
}
?>
Loaded from a File
If you are doing testing without loading a page from a server path (e.g. localhost, 127.0.0.1, 192.168.xxx.yyy, etc) then the only way to make a browser load an XHTML application and use the XML parser is to give the file an .xhtml extension; example.xhtml.
XHTML / XML Parser Browser Support
Browsers with XHTML support via XML parsers include Internet Explorer 9+ (XML parser support in older versions do in a very convoluted fashion support application/xml), Mozilla Suite 0.8+ (all Firefox versions), Opera 7+, and early versions of KHTML (Konqueror and therefore all versions of Safari and by further extension Chromium/Chrome).
