The HTML5 spec says, of the HTMLStyleElement's scoped attribute:

"If the scoped attribute is present, then the user agent must apply the specified style information only to the style element's parent element (if any), and that element's descendants."

The Shadow DOM spec says, of style elements:

"Conversely, to enforce lower-boundary encapsulation, CSS rules declared in a shadow root style sheets must not apply in the document tree, with two exceptions: 1. ... 2. The @host @-rule matches a shadow host in the nesting tree."

Given an element with this in a shadow tree:

<style scoped>
@host {
  * { border: 1px solid salmon; }
}
</style>

then either the host element has salmon borders (ie the Shadow DOM spec wins) or it doesn't (ie the HTML5 spec wins.)

The Shadow DOM spec should clarify which it is.