The closure of XHTML elements
If you have followed the crowd and abandoned HTML for XHTML, you may have noticed that some elements need to be closed in your markup. The reason is that XHTML is simply a reformulation of HTML in XML syntax. It’s a subtle but very significant difference between HTML and XHTML.
The MIME type of a document is the only thing that controls whether it is HTML or XHTML. Since browsers don’t fully support XML mode, full XHTML processing is not an option. This leads to markup being served as “text/html” whether you call it XHTML or not.
Unlike CSS, there is no shorthand notion in HTML. Consider the following code:
The slash at the end closes the element. But the following is not valid:
Neither is this:
The self-closing slash character is interpreted as an attribute in the markup and ignored (since it’s not a legal attribute name). This means that the self-closed div actually becomes an opening tag without a closing tag.
So why does the self-closed img element work? It is because the HTML DTD states:
As you can see, it allows for empty content. On the other hand, div elements are not allowed to have empty content.
This is just the tip of the iceberg. For more information, have a look at the infamous Appendix C of the XHTML 1.0 standard.