Development: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>,
and so on.

Styling

Make sure you provide enough CSS ids and classes to enable a designer to modify everything with CSS.
Never use inline styles
If you need to make basic style definitions for a module, 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: Development:Navigation_2.0

Documentation