However, I personally feel that specifying unique content information in an attribute of an element is more “natural”. To me, using the class just feels “wrong” (even though it is definitely more correct from a standards point of view).

class=”default_foo” may make more semantic sense to a computer, but to a developer I think it is more confusing than default=”foo” (especially when taken in the context of a text field). This most likely is the result of the connotation of “class”, which doesn’t really fit well with providing configuration for this specific example.

This, of course, is just my opinion. Regardless, you are correct that it is not strictly valid nor standard HTML.

This should be separated out into JavaScript completely and add a CSS class as the hook.