reset styles for a:not([href])

See twbs/bootstrap#19411 for reference

Having different styles for <a> with and without href is the default
browser behavior.

We could also do link styling on a[href], but that would be hard to
overwrite.

The downside of this solution is that it is now hard to overwrite
a:not([href]), but that is a much less common task.

Author: Tobias Bengfort

sass/base.scss

@@ -44,6 +44,15 @@ a {
     }
 }
 
+a:not([href]) {
+    color: inherit;
+    text-decoration: inherit;
+
+    &:hover, &:focus, &:active {
+        color: inherit;
+    }
+}
+
 *::selection {
     background: $color-link;
     color: $color-bg;