Add a way to share styles between elements and the global scope

[sorvell]

Given that a user has made a custom-style for something like layout, for example, it's convenient to be able to use this inside an element as well. Currently, this requires code duplication.

If we supported a new syntax like this: <link rel="import" is="custom-stylesheet"> then the style could be defined once and used globally (via cusotm-style functionality) or locally inside an element via the link's location inside an element <dom-module> (custom-style uses this technique now). If we do this, we should deprecate the current syntax for loading remote stylesheets in element (<link rel="import" type="css">)

[eximius313]

+1

[adalinesimonian]

+1

[ebidel]

/sub

[agektmr]

+1

[sorvell]

Changes that will be merged for this issue are here: #2198.

[sorvell]

@arthurevans can you please close this when you feel this has been adequately documentated? Thanks.

[arthurevans]

Documented, as described in #2229.