HTML lexer doesn't handle elements with dashes in tag name
The html lexer generates
<span class="err">'s on closing elements with dashes in the name, e.g. You can see this in the following inline example that's using (presumably) the same HTML lexer:
Here's the the same code without specifying a lexer:
This isn't a standard HTML tag, but custom elements/web components will have dashes in their names (e.g, http://www.x-tags.org/index).