style: Implement the body text color quirk in a more straight-forward way.

This avoids grabbing the document when values that inherit from the body,
whatever that means, aren't under the body.

In that case we'll get a semi-random value, but that's also mishandled by Gecko
anyways (and probably Blink, though haven't tested), and doesn't really make
much sense.

components/style/dom.rs

@@ -727,6 +727,10 @@ pub trait TElement : Eq + PartialEq + Debug + Hash + Sized + Copy + Clone +
         override_lang: Option<Option<AttrValue>>,
         value: &PseudoClassStringArg
     ) -> bool;
+
+    /// Returns whether this element is the main body element of the HTML
+    /// document it is on.
+    fn is_html_document_body_element(&self) -> bool;
 }
 
 /// TNode and TElement aren't Send because we want to be careful and explicit