Consider adding a specific styling API to better integrate with constructible stylesheets

[sorvell]

Constructible stylesheets are soon coming to the web platform. They should provide a significant performance improvement for styling Shadow DOM!

The intention of the API is to allow sharing of stylesheets between shadowRoots that use the same styling, and this is a perfect fit for custom elements. It does strongly discourage modifying styles per instance since the stylesheet would be shared across all elements of a given type.

Because of this, it may not make sense for users to put non-dynamic <style> elements into the render method since it looks like these styles can change. Although this is not the case when shady-render is used, that's a polyfill specific limitation rather than part of the core design.

Perhaps a static get style() could be added which is how users declare element styling.

If so, we either need to integrate this feature with shady-render or stop using it.

We should probably consider if this is worth doing prior to a 1.0 release.

[daKmoR]

+1 for static get style()

we are using something like this for a while now and it's really nice to work with :)

we sort of have a variation of https://github.com/lit-styles/lit-styles/tree/master/packages/styled-lit-element - maybe get some inspiration from it :)