HTML and XHTML Pocket Reference, 3rd Edition

How XHTML Differs from HTML

Because XHTML is an XML language, its syntax is more strict and differs from HTML in these key ways:

• All element and attributes must be lowercase. For example, <a href="example.com"> . . . </a>.

• All elements must be terminated--that is, they must include an end tag. For example, <p> . . . </p>.

• Empty elements must be terminated as well. This is done by including a slash at the end of the tag. A space is commonly added before the slash for backward compatibility with older browsers. For example, <hr />, <img />, <meta />.

• All attribute values must be contained in quotation marks (either single or double). For example, <td colspan="2">.

• All attribute values must be explicit and may not be minimized to one word, as is permitted in HTML. For example:

  • checked="checked"

  • selected="selected"

  • multiple="multiple"

• Nesting restrictions are more strictly enforced. These restrictions are explicitly stated:

  • An a element cannot contain another a element.

  • The pre element cannot contain img, object, applet, big, small, sub, sup, font, or basefont.

  • The form element may not contain other form elements.

  • A button element cannot contain a, form, input, select, textarea, label, button, iframe, or isindex.

  • The label element cannot contain other label elements.

• The special characters <, >, &, ', and " must always be represented by their character entities, including when they appear within attribute values. For example, <, >, &, ', and " (respectively).

• In HTML, the name