What is this?

HTLML5 can be “serialized” (delivered to browsers) as either HTML or XHTML. The HTML serialization is compatible with the syntax rules for HTML 4 and earlier, whereas the XHTML serialization is based on XML, the Extensible Markup Language, which has very strict syntax rules.

The more relaxed rules for HTML are very convenient in many ways, but XHTML has a significant advantage: the browser is obliged to to report any syntax errors it finds and to stop processing the web page whenever if finds one. This is rather draconian behavior on the Web, where the goal is to communicate easily, not to display pedantic errors to a site’s visitors. But adhering to strict syntax rules means that there can be nothing ambiguous about the structure of a page–nothing that different browsers might interpret differently.

The audience for this web page is students who are taking college-level web design courses offered by a computer science department. In this context, using XHTML instead of HTML has the additional value of getting students accustomed to strict syntax rules, which are the norm in most coding languages used in computer science.

What about this web page?

We begin with some information about how your browser and this web page interacted. In order for a web page to be XHTML5, it must be delivered to the browser as an XML document using the application/xml or application/xhtml+xml MIME type. But browsers that do not “understand” these MIME types typically simply display the code of XML documents instead of rendering them as web documents. So this page includes server-side code that tested whether or not your browser announced that it could understand xhtml or xml documents. Based on that test, the page was delivered to you browser as either an XHTML5 document or as a plain HTML document. The next two paragraphs summarize what was found:

Your browser’s Accept header was:

This page was delivered with MIME type:

And, just to see whether your browser implements common (X)HTML5 elements, here is a how your browser displays an empty HTML5 canvas element, which should show up as a blue-gray rectangle:

Top of Document: PHP, DOCTYPE, and html Code

Here is the code you can past into the beginning of an XHTML5 document so that it will actually be delivered as XHTML5 to those browsers that support XHTML, but will fall back to HTML for browsers that do not support XHTML.

In order for this code to work, your server must support the PHP server-side scripting language, and your web page must have a file extension that your web server will recognize as one that contains PHP code. If you have PHP installed, .php is undoubtedly a safe file extension. In our lab, our Apache web server has been configured to recognize both .php and .xhtml as extensions for files that contain PHP code. (Note that this web page’s extension is .xhtml.)

The last two lines of the code include the usual HTML5 doctype and the html tag that starts the document proper. The xmlns attribute in the html tag is needed for the page to be properly recognized as XHTML. Some testing (admittedly not exhaustive) indicates that this attribute has no effect in browsers that do not accept proper XHTML.

Validation Links

All code for this course must pass the W3C validation suites for HTML5 and CSS3 with no errors and no warnings (except that the validator currently issues a warning that HTML5 is experimental; that one warning is acceptable). Paste the following code at the bottom of each of your web pages to give them links to the two validators for easy checking. Note that the code ends with the /body and /html tags that are needed to balance the html tag at the beginning of your web page and the body tag that comes after the head element of the document.