Apart from the issue regarding attributes, I do agree adding an attribute on the element isn't great. Though mostly because this is a functionally a checkbox, as evidenced by how they're fine with it falling back to a checkbox, and so they could have just added a CSS attribute instead.

> I believe certain doctypes or serving the HTML with another content type (XML) will throw an error for not having the quotes.

That is irrelevant - most HTML found in the wild is not particularly close to valid XML as it is. And as per sibling comments, there's no need for quotes here for html - they are multiple separate boolean attributes, and the norm in HTML is to use those without a value for the truthy case.