Here, Brian proposes a kind of minimum viable web component that handles logic like keyboard control and accessibility, but leaves the styling practically untouched. Check out his panel-set demo of a tabbed interface.
I really, really like the way that it wraps existing content. If the web component fails for any reason, the content is still available. So the web component is a progressive enhancement:
An experimental custom element that wraps plain-old HTML (view the source) and decorates function, keyboard handling, accessibility information.