Created attachment 193592 [details] reduction An @host rule can match a node without a shadowRoot. This behavior violates the shadowDOM spec which says, "The declarations of the rules in a @host @-rule must only be matched against the shadow host of the shadow tree in which the style is specified..." (https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/shadow/index.html#host-at-rule). If a div is given a shadowRoot containing an @host rule that uses the * selector specifying a set of styles, that div and all subsequent divs will have the specified set of styles applied. According to the spec, only the div with the shadowRoot should have the @host styling applied to it.