This issue was reported by @domenic here, and is being moved to the OpenUI repo.

The x=x pattern in HTML is generally used as a way to allow XML serializations (i.e. "XHTML") to do boolean attributes. I.e. instead of the simple <div checked>, you'd do <div checked="checked">, because XML does not allow value-less attributes.

We should probably avoid using this pattern for a non-boolean attribute, as it will confuse people.

Ideas:

• Rename the attribute. (E.g. toplayerbehavior=popup or toplayer=popup.)
• Rename the value. (E.g. popup=default, popup=onlyone.)
• Have no value, and just a state. (So you'd do <div popup> with no value to get this behavior.)

I imagine there's going to be a general bikeshedding fest on popup/hint/async, but I just wanted to note this as a relatively high-priority consideration in that bikeshed.