Are self-closing input tags valid in HTML 4?
The syntax is valid in some places but doesn't mean the same as in XHTML, so don't use them.
In HTML 4 <foo /> (where foo is the name of an element defined as EMPTY) means the same as <foo>> which means the same as <foo>> (although almost no browser supports the syntax correctly, Emacs-W3 used to, but broke compatibility with the standard in favour of rendering so-called HTML compatible XHTML 1.0 documents correctly).
This is, therefore, valid in places where you can have a > such as anywhere you are allowed an <img> but not in other places (such as an <hr> that is a child element of the <body> (in Strict)).
The interaction with the rules for optional start and end tags adds more complication. In a Transitional document, this is valid:
<link …/>
<h1>Hello, world</h1>
and means:
<link>
</head>
<body>
>
<h1>Hello, world</h1>
This shorthand syntax could be useful, or at least a time saver, for things like:
<title/The quick brown fox/
instead of the more verbose:
<title>The quick brown fox</title>
… but the syntax has never been well supported and the specification says it should be avoided.
Solved actually... according to W3C HTML 4 validator it's better not to use this style of writing element names in HTML 4:
NET-enabling start-tag requires SHORTTAG YES
<br />
The sequence <FOO /> can be interpreted in at least two different ways,
depending on the DOCTYPE of the document. For HTML 4.01 Strict, the '/' terminates
the tag <FOO (with an implied '>'). However, since many browsers don't interpret it
this way, even in the presence of an HTML 4.01 Strict DOCTYPE, it is best to avoid
it completely in pure HTML documents and reserve its use solely for those written in XHTML.