HTML
One thing I like about XML, and in particular, that XML introduced is —
XML introduced a notation for self-closing (i.e., void) tags.
I.e., the trailing slash
For example:
<img src="..." />
(Note the "/>" at the end of the tag.)
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From the point of view or someone writing a parser, I liked this convention.
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I think this would have also been a great UX for custom HTML elements, too. As it would make it so wouldn't have to always include a closing tag.