Web Design in a Nutshell, 3rd Edition

Element Termination

In HTML, it is okay to omit the end tag for certain block elements (such as p and li). The beginning of a new block element is enough to trigger the browser to parse the previous one as closed. Not so in XHTML. To be well-formed, every container element must have its end tag, or it registers as an error and renders the document noncompliant.