The purpose of Open UI to the web platform is to allow web developers to style and extend built-in web UI controls, such as <select> dropdowns, checkboxes, radio buttons, and date/color pickers.
To do that, we'll need to fully specify the component parts, states, and behaviors of the built-in controls, as well as necessary accessibility requirements, and provide test suites to ensure compatibility. We'll also implement polyfills for our extensible web UI controls.