Use empty pseudo to cascade only inheritable properties for text

Since for nested inline elements non-inheritable properties are
properly stored in the inline context of an inline fragment, so get
rid of them on the style.

resources/servo.css

@@ -168,6 +168,12 @@ svg > * {
   display: none;
 }
 
+/* style for text node. */
+*|*::-servo-text {
+    margin: 0;
+}
+
+/* style for text in input elements. */
 *|*::-servo-input-text {
     margin: 0;
 }