Comment by marianoguerra | Hacker News Reader

Comment by marianoguerra | Hacker News Reader

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I found some references here:

"Custom elements cannot be self-closing because HTML only allows a few elements to be self-closing"

https://developers.google.com/web/fundamentals/web-component...

It seems only void elements are allowed to be self closing:

https://html.spec.whatwg.org/multipage/syntax.html#void-elem...

The trailing U+002F (/) in a start tag name can be used only in foreign content to specify self-closing tags. (Self-closing tags don't exist in HTML.) It is also allowed for void elements, but doesn't have any effect in this case.

https://html.spec.whatwg.org/multipage/parsing.html#parse-er...

The important bit here being that the custom element API simply doesn't allow declaring all relevant features of elements except their names.

Whereas (SGML) DTDs such as in any DTD for HTML before HTML5, elements were declared like this

    <!element img - - EMPTY>

for elements with declared content EMPTY ("void elements" in HTML5 parlance), and

    <!element html O O
     (head,body)>

as a basic example for tag omission/inference, where given these tag omission indicators (the letter O in the respective position), you can omit the html tags (and also head, and many other start- and end-element tags) when SGML can unambiguously infer those for you.

Valid custom elements must also contain a dash.

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