This document outlines the Well Formedness Constraints in ASTv2.ebnf.

Element Type Match

The literal characters of the HtmlTagName that appear in the opening and closing of a SimpleElement (before the first whitespace character) must be identical.

Component Tag Match

The literal characters of the ExpressionNode that appear in the opening and closing of an InvokeComponent (before the first whitespace character) must be identical.

Block Tag Match

The literal characters of the ExpressionNode that appear in the opening and closing of an InvokeComponent (before the first whitespace character) must be identical.

This might not be literally true, and we should try to figure out what the Handlebars parser does here