The simple theme renders "bare bones" HTML elements. The simple theme is most often used as a starting point for other themes. (See Extending Themes for more.)

For example, the textfield tag renders the HTML <input/> tag without a label, validation, error reporting, or any other formatting or functionality.

Both the xhtml theme and css_xhtml theme extend the simple theme. Look to them for examples of how to build on the foundation laid by the simple theme.

Head Tag

The simple theme head template prints out a javascript include required for the datetimepicker tag to render properly.