The "selecting through shadows selector" (also referred to "shadow-piercing descendant combinator") allows selecting elements within regardless of them being in a ShadowRoot or nested ShadowRoot or not. It is somewhat similar to ::shadow . Note: The ">>>" selector used to be called "/deep/" (and is currently implemented in Chrome as "/deep/". Link to specification for reference: http://dev.w3.org/csswg/css-scoping-1/#deep-combinator  https://bugzilla.mozilla.org/show_bug.cgi?id=1112251
We should not implement this until there's agreement that this is actually desired. See https://github.com/w3c/webcomponents/issues/78.
It seems that Chrome 60 will be treating the >>> combinator as the descendant combinator, so perhaps it would be wise to simply follow suit? https://developers.google.com/web/updates/2017/06/chrome-60-deprecations#make_shadow-piercing_descendant_combinator_behave_like_descendent_combinator