Basic (quite basic indeed) Shared Stylesheet for elements and pages

[bennypowers]

... If the failover to Helvetica, Arial, sans-serif is enough to cover the correct character rendering for other languages then it wouldn't be required per say. I think what I'm after is should "we" be required to show a complete example of what "should" be done in our demos. Noto Hebrew will not load otherwise in the RTL demos that use Hebrew text.

This should be a separate PR. I think it should be part of a base style sheet, and that stylesheet should be shared (i.e. adopted) into each element's shadow root as well. This is a separate issue and will require discussion. The first iteration should be quite minimal - lang, h1-6, p, a, etc. - no classes.

Originally posted by @bennypowers in #2106 (comment)