XHTML

XHTML Strict 1.0

Moodle output must be compliant with XHTML Strict 1.0. This means it must be:

Well-formed XML

In a well-formed XML document, every opening tag has a matching close tag; tags are properly nested attributes are properly quoted, and the file contains no syntax errors. See the XML specification for a formal definition.

While developing, you should have the option Administration ► Server ► Debugging ► XML strict headers turned on. With this option on, your web browser will refuse to display any page that is not well-formed. This makes such problems easy to find and fix.

Valid XHTML Strict

This means that the XML of your page follows the particular rules from the XHTML-1.0-Strict DTD. For example, the first tag in the file must be <html>, a <form> tag must have an action="" attribute, an <li> can only appear inside an <ol> or <ul>, you cannot use <frame> tags, and so on. and so on.

You can check whether the HTML you output is valid by using a HTML validator, for example the Html Validator add-on for Firefox.

Semantic markup

That is, HTML tags should be used only to mark up the appropriate types of content. For example:

tables should not be used for page layout, just to display tabular information,
if something is a heading, it should be marked up using <hn> tags, for an appropriate n,
if something is a list, it should marked up using <ol>, <ul> or <dl>,
see Semantic HTML for further details

Styling

How the HTML is laid out should be controlled by CSS in whichever theme is currently selected.

Ensure that the HTML contains enough id="..." and class="..." attributes so that theme designers can easily control how
Never embed inline styles in the HTML (that is, do not use the style="..." attribute).
As you change core Moodle code, you must update the standard theme so that Moodle looks OK out of the box.
If you need to make basic style definitions for a module (or some other sorts of plugin), put them in a file called styles.php in that module. This will be included into every theme.

Moodle API

Use the functions in lib/weblib to do as much as possible (print_header(), print_box() etc)
This API will change a lot in Moodle 2.0. See: Navigation_2.0

Documentation