ControlUICustomization - Common HTML structure for views

In the Open Questions you raise the following issue:

Allowing the developer to clear the default styles and provide their own stylesheet isn't a solution, because different platforms may have used a different HTML structure to build their control views. E.g. the slider track might be built with styled <div>s in one platform and styled <span>s in another.

I'm so glad you pointed this out because I incorrectly assumed that the complete HTML structure of each control would be fully specified.

I strongly believe that unless the entire structure of the contro is specified completely this entire specification will end up as an interoperability nightmare and everyone will stick to writing custom controls.

So the same developer-provided stylesheet would not work interoperably unless the entire control UI was replaced.

This is the core of the problem, it puts the onus of interoperability on the developer instead of on the browser.

Start with the Open Questions passage quoted in the issue and review the interoperability concern about platform-specific control HTML. Determine whether the proposal specifies a common structure for each control and what concrete specification change would resolve the concern; the work is done when the required structure and interoperability outcome are unambiguous.