This should be a warning, not an error, for empty elements.

From html5.dtd:

Fromhttp://www.w3.org/html/wg/drafts/html/master/syntax.html#self-closing-start-tag-state:

">" (U+003E)
Set the self-closing flag of the current tag token. Switch to the data state. Emit the current tag token.

Anything else gives a parse error.