In web page design, and generally for all markup languages based on SGML, such as HTML, XML, a well-formed element is one that is either

• opened and subsequently closed,
• an empty element, which in that case must be terminated,

and at the same time also is

• properly nested so that it does not overlap.

For example, in HTML: word is a well-formed element, while word is not, since the bold element is not closed. Empty elements (tags without text in between) can be closed by putting a slash at the end of the tag, e.g. ,
,