How does empty start tag work in HTML4?

The HTML4 specification mentions various SGML shorthand markup constructs. While I understand what others do, with a help of HTML validator, I cannot find understand why anyone would want an empty start tag. It cannot even have attributes, so it's not a shorter .

Solution

The SGML definition of HTML4 enables the empty start feature. In it, there is an interesting section with features.

FEATURES
  MINIMIZE
    DATATAG  NO
    OMITTAG  YES
    RANK     NO
    SHORTTAG YES
  LINK
    SIMPLE   NO
    IMPLICIT NO
    EXPLICIT NO
  OTHER
    CONCUR   NO
    SUBDOC   NO
    FORMAL   YES
  APPINFO NONE

The important section of features is MINIMIZE section. It enables OMITTAG which is a standard feature of HTML, which allows start or end tags to be ommited. This is particular allows you to write code like  a b, without closing paragraphs.

The more important part is SHORTTAG feature, which is actually a category. However, because it's not expanded, the SGML automatically assumed YES for all entries in it. It has the following categories in it. Feel free to skip this list, if you aren't interested in other shorthand features in SGML.

ATTRIB, which deals with attributes, and has following options.
- DEFAULT - defines whether attributes can contain default values. This allows writing  without defining every single attribute. Nobody would want to write  after all. Hey, I even gave up trying to write all that. This is a commonly supported feature.
- OMITNAME - if the attribute and value have the same name, the value is optional. This allows writing <input type="checkbox" checked> for instance. This is a commonly supported feature (although, HTML5 defines default to be empty string, not an attribute name).
- VALUE - allows writing values without quotes. This allows writing code like  for instance. This is a commonly supported feature.
ENDTAG, which is a category for end tags containing the following options.
- UNCLOSED - allows starting a new tag before ending the previous tag, allowing code like </b.
- EMPTY - allows unnamed end tags, such as something</>. They close most recent element which is still open.
STARTTAG, which is a category for start tags containing the following options.
- NETENABL - allows using Null End Tag notation. It's worth noting this notation is incompatible with XHTML. Anyway, the feature allows writing code like <b/<i/hello//, which means the same thing as hello.
- UNCLOSED - allows starting a new tag before ending the previous tag, allowing code like <p.
- EMPTY - this is the asked feature.

Now, it's important to understand what EMPTY does. While <> may appear useless at first (hey, how could you determine what it does, when nothing aside of Validator supports it), it's actually not. It opens the previous sibling, allowing code like the following.

<ul>
  <li class=plus> hello world
  <> another list element
  <> yet another
  <li class=minus> nope
  <> what am I doing?
</ul>

In this example, the list has two classes, plus and minus for positive and negative arguments. However, the webmaster was lazy (and doesn't care about that HTML4 doesn't support this), and decided to use empty start tag in order to not specify the class for next elements. Because <li> has optional end tag, this automatically closed previous <li> tag.