To enforce upper-boundary encapsulation, CSS rules declared by the document must not apply in a shadow DOM subtree, unless the apply-author-styles flag is set for this subtree. The flag signals that document CSS rules are applicable in the shadow DOM subtree.
Conversely, to enforce lower-boundary encapsulation, CSS rules declared in a shadow DOM subtree must not apply in the document tree."
"7 Styles
To enforce upper-boundary encapsulation, CSS rules declared by the document must not apply in a shadow DOM subtree, unless the apply-author-styles flag is set for this subtree. The flag signals that document CSS rules are applicable in the shadow DOM subtree.
Conversely, to enforce lower-boundary encapsulation, CSS rules declared in a shadow DOM subtree must not apply in the document tree."