[webcomponents] [Shadow] Section 2.4 - CSS inherit by default? (#314)

The spec says: 
["In resolving CSS inheritance, an element must inherit from the parent node in the composed tree, if applicable."](https://w3c.github.io/webcomponents/spec/shadow/index.html#composed-trees)

This implies that, in the general case, all CSS properties that inherit by default will automatically bridge shadow dom boundaries, and that the use of the CSS value ```inherit``` will potentially pull styling from across shadow dom boundaries as well.

Is this the behavior that we want? Clearly, for our styling story, we *do* want CSS custom properties to inherit through shadow dom boundaries, but this should be the exception, not the default.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/314