Why don't self-closing script elements work?

The non-normative appendix ‘HTML Compatibility Guidelines’ of the XHTML 1 specification says:

С.3. Element Minimization and Empty Element Content

Given an empty instance of an element whose content model is not EMPTY (for example, an empty title or paragraph) do not use the minimized form (e.g. use <p> </p> and not <p />).

XHTML DTD specifies script elements as:

<!-- script statements, which may include CDATA sections -->
<!ELEMENT script (#PCDATA)>

To add to what Brad and squadette have said, the self-closing XML syntax <script /> actually is correct XML, but for it to work in practice, your web server also needs to send your documents as properly formed XML with an XML mimetype like application/xhtml+xml in the HTTP Content-Type header (and not as text/html).

However, sending an XML mimetype will cause your pages not to be parsed by IE7, which only likes text/html.

From w3:

In summary, 'application/xhtml+xml' SHOULD be used for XHTML Family documents, and the use of 'text/html' SHOULD be limited to HTML-compatible XHTML 1.0 documents. 'application/xml' and 'text/xml' MAY also be used, but whenever appropriate, 'application/xhtml+xml' SHOULD be used rather than those generic XML media types.

I puzzled over this a few months ago, and the only workable (compatible with FF3+ and IE7) solution was to use the old <script></script> syntax with text/html (HTML syntax + HTML mimetype).

If your server sends the text/html type in its HTTP headers, even with otherwise properly formed XHTML documents, FF3+ will use its HTML rendering mode which means that <script /> will not work (this is a change, Firefox was previously less strict).

This will happen regardless of any fiddling with http-equiv meta elements, the XML prolog or doctype inside your document -- Firefox branches once it gets the text/html header, that determines whether the HTML or XML parser looks inside the document, and the HTML parser does not understand <script />.

Internet Explorer 8 and earlier do not support XHTML parsing. Even if you use an XML declaration and/or an XHTML doctype, old IE still parse the document as plain HTML. And in plain HTML, the self-closing syntax is not supported. The trailing slash is just ignored, you have to use an explicit closing tag.

Even browsers with support for XHTML parsing, such as IE 9 and later, will still parse the document as HTML unless you serve the document with a XML content type. But in that case old IE will not display the document at all!

The people above have already pretty much explained the issue, but one thing that might make things clear is that, though people use <br/> and such all the time in HTML documents, any / in such a position is basically ignored, and only used when trying to make something both parseable as XML and HTML. Try <p/>foo</p>, for example, and you get a regular paragraph.

Unlike XML and XHTML, HTML has no knowledge of the self-closing syntax. Browsers that interpret XHTML as HTML don't know that the / character indicates that the tag should be self-closing; instead they interpret it like an empty attribute and the parser still thinks the tag is 'open'.

Just as <script defer> is treated as <script defer="defer">, <script /> is treated as <script /="/">.

Internet Explorer 8 and older don't support the proper MIME type for XHTML, application/xhtml+xml. If you're serving XHTML as text/html, which you have to for these older versions of Internet Explorer to do anything, it will be interpreted as HTML 4.01. You can only use the short syntax with any element that permits the closing tag to be omitted. See the HTML 4.01 Specification.

The XML 'short form' is interpreted as an attribute named /, which (because there is no equals sign) is interpreted as having an implicit value of "/". This is strictly wrong in HTML 4.01 - undeclared attributes are not permitted - but browsers will ignore it.

IE9 and later support XHTML 5 served with application/xhtml+xml.

Difference between 'true XHTML', 'faux XHTML' and 'ordinary HTML' as well as importance of the server-sent MIME type had been already described here well.

If you want to try it out right now, here is simple editable snippet with live preview including self-closed script tag (see <script src="data:text/javascript,/*functionality*/" />) and XML entity (unrelated, see &x;).

As you can see, depending on the MIME type of embedding document the data-URI JavaScript functionality is either executed and consecutive text displayed (in application/xhtml+xml mode) or not executed and consecutive text 'devoured' by the script (in text/html mode).

div { display: flex; }
div + div {flex-direction: column; }

<div>Mime type: <label><input type="radio" onchange="t.onkeyup()" id="x" checked  name="mime"> application/xhtml+xml</label>
<label><input type="radio" onchange="t.onkeyup()" name="mime"> text/html</label></div>
<div><textarea id="t" rows="4" 
onkeyup="i.src='data:'+(x.checked?'application/xhtml+xml':'text/html')+','+encodeURIComponent(t.value)"
><?xml version="1.0"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"
[<!ENTITY x "true XHTML">]>
<html xmlns="http://www.w3.org/1999/xhtml">
<body>
  <p>
    <span id="greet" swapto="Hello">Hell, NO :(</span> &x;.
    <script src="data:text/javascript,(g=document.getElementById('greet')).innerText=g.getAttribute('swapto')" />
    Nice to meet you!
    <!-- 
      Previous text node and all further content falls into SCRIPT element content in text/html mode, so is not rendered. Because no end script tag is found, no script runs in text/html
    -->
  </p>
</body>
</html></textarea>

<iframe id="i" height="80"></iframe>

<script>t.onkeyup()</script>
</div>

You should see Hello, true XHTML. Nice to meet you! below textarea.

For incapable browsers you can copy content of the textarea and save it as a file with .xhtml (or .xht) extension (thanks Alek for this hint).

Simply modern answer is because the tag is denoted as mandatory that way

Tag omission None, both the starting and ending tag are mandatory.

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/script