On the various meanings of <tag />

With the growth of XHTML, some HTML documents have started sprouting things of the form '<tag />' or '<tag/>', intended to represent empty tags. Sometimes this is due to attempts at XHTML to HTML compatibility, and sometimes it's just due to HTML superstitions. (Most people who write HTML are not experts and have not read the W3C specifications, so superstitions are not uncommon.)

These people and documents are making a mistake. Unfortunately, there are three entirely separate things that <tag /> can mean:

• if you are actually successfully serving XHTML, this is an empty (self-closed) element. (Hopefully it is an element that can actually be empty.)

• in HTML in all the major web browsers, this means 'tag, with an attribute called "/"'.

• in strictly compliant HTML, including as parsed by the W3C validator, this means '<tag>>', because it is actually a SGML feature called minimization.

The difference between validators and browsers is an especially pernicious gotcha; a web page can pass validation, but in a completely different way than how a browser will interpret it. (It happens because validators usually use a real SGML parser, completely with all of the obscure features, but browsers do not.)

It is very fortunate that no popular browser actually implements tag minimization, because no one expects them to. Also, HTML has some remarkably dark corners where you can get eaten by a grue.

If you want to read more about this, Empty elements in SGML, HTML, and XHTML will give you all of the detail and background that you could want. (There you will also find a nice example of why you should always put attribute values in quotes.)

(I owe my exposure to these pages and this issue to plasmaturm.org and Frank Hecker. And in truth in advertising, the final impetus for writing this down was provided by this blog entry.)