This issue should land on CSS Scoping spec, but I think it's better to discuss here and have some level of common understanding before we go www-style, because complex tree of trees is too much for most members at www-style. If this doesn't look right, please close and I'll go www-style.

The Shadow Cascading of CSS Scoping spec[1] defines:
 * Outer wins if inner/outer
 * Younger wins if younger/older

But Takashi and I are finding a few cases where trees have more complex relationships. We should define how cascading occurs in such cases, which wins, or prohibit.

Case 1: Re-distributions
By using multiple and nesting together, re-distribution can cause such relationship. Example:
http://jsbin.com/wabigi
div
  div
  SR-OLDER
    div
      content
      SR-NEST-OF-OLDER
        style ::content * {color:red}
        content
  SR-YOUNGER
    style ::content * {color:green}
    shadow

Case 2: :host and shadow-piercing descendant combinator
The combination of :host and shadow-piercing descendant combinator can select descendants of younger/older trees.

[1] http://dev.w3.org/csswg/css-scoping/#cascading