Why can't I say <iframe /> and be done with it?

Question

I thought that in XML you could have a tag with opening and closing tags, and then if you didn't need the closing tag, you could just use a tag with a slash at the end of it.

<!-- needs text/elements within tag so opening and closing tag are needed -->
<myTag>Text here</myTag>
<!-- doesn't need text/elements so doesn't have closing tag -->
<myTag /> 

I'm guessing that this is just because that's whatever everyone decided was best and this is now just a XHTML peculiarity, but why? Is there a technical reason for this that I don't know about?

Thanks!

Answer 1

It's because the XHTML DTD says so :)

Only these elements are allowed to be empty:

• area
• base
• br
• col
• hr
• img
• input
• link
• meta
• param

For example link is declared as:

<!ELEMENT link EMPTY>

whereas script and iframe are declared as:

<!ELEMENT script (#PCDATA)>

and

<!ELEMENT iframe %Flow;>
<!ENTITY % Flow "(#PCDATA | %block; | form | %inline; | %misc;)*">
...

Answer 2

If browsers treated XHTML as XML, you would be right. But they treat it as HTML, where the rules are different.