I have mentioned here and there that XHTML (and XML in general) wasn’t designed to support SGML null end tags. This isn’t completely true. XML supports a restricted and altered form of null end tags, and in fact they are used all the time.
Null end tags are a way to abbreviate an end tag to a single character. They are not supported by most common HTML user agents, but they do exist in HTML’s profile of SGML and there are HTML user agents that support them. In HTML and the default SGML profile, null end tags look like this:
<title/This is the title of the page/
For fully compliant HTML user agents, the above is equivalent to the following:
<title>This is the title of the page</title>
As you can see, the contents of the element are surrounded by “/” characters, which are used more or less like the quotes used for attribute values. If an SGML profile and DTD requires a certain element’s end tag to be omitted, only one slash is relevant for the element (any further slashes will be treated as character data). For example, the following tag is valid in HTML:
<img src="image.png" alt="An image"/
Although it doesn’t save any characters and isn’t widely supported, it is perfectly legal according to the standard. This is where I have discussed problems with XHTML. The above isn’t legal in XHTML, but the following is the closest equivalent:
<img src="image.png" alt="An image"/>
An XHTML user agent would see the above as a single img tag, but a fully compliant HTML user agent would see it as a shortened null end tag like the previous example but with a “>” character after it. The “>” character would be seen as regular character data and would display on the page itself. Despite common practice, a space before the “/” character wouldn’t change this.
I have said that this issue is due to XHTML/XML not supporting null end tags. However, it’s more accurate to say that XML doesn’t support null end tags in the same way as HTML. Rather than the contents being surrounded by two slashes, they are surrounded by a slash and a greater-than sign (/ ... >) with the additional constraint that they may only be used when the contents are empty. So the second img example is actually XML’s version of null end tags: the start tag ends with the “/” character and the end tag is represented by the “>” character. Because end tags may not be omitted in XML, the “>” character is always required, and because the null end tag rule in XML is defined as “IMMEDNET” (explained below), it must close immediately after it is started, so there may be no actual content.
Although the specifications don’t clearly discuss these issues, they are a result of the respective standards’ SGML declarations that define the profile of SGML used. See the HTML SGML declaration and the XML SGML declaration. These declarations are automatically assumed by the browser when they are given hint to treat the page as HTML or XML (such as via the content type). The SGML declaration defines the most basic level of how the SGML document is written. It defines which characters define tags, marked sections, character references, processing instructions, etc., what kinds of shorthand features may be used, possibly some default character entities that are available regardless of the DTD, and other lexical aspects of the document. The SGML declaration is applied on top of a default profile, called the “reference concrete syntax” that is defined in the SGML standard itself.
The HTML SGML declaration isn’t very big because it mostly uses SGML’s defaults. The defaults include “/” for syntax.delim.net, meaning that null end tag contents are delimited by “/” characters. XML uses “>” for syntax.delim.net, plus “/” for syntax.delim.nestc. Nestc is an extension to the original SGML standard that provides a different value for the null end tag delimiter that finishes the start tag. XML uses other extensions, such as more specific options in the “features” section. HTML enables features.minimize.shorttag, which allows shorthand constructs like null end tags, while XML specifically has features.minimize.shorttag.starttag.netenabl set to “IMMEDNET” which, as mentioned above, enables null end tags with the restriction that they must close immediately after opening.
The reason XML was designed to support this form of null end tags was to reduce the potential clutter caused by a large number of empty elements. The null end tag delimiters were altered so that the null end tags don’t look too alien for people who are used to the widely supported parts of the HTML standard. They were designed to look like regular start tags with a simple slash before the end, which reminds people of the function of end tags. In this way, they managed to design XML to be strict, efficient, intuitive, and compatible with modern SGML user agents that know where to find the SGML declaration.