Meta: export the fullscreen element concept (#151)

Fixes #150.

Author: foolip

fullscreen.bs

@@ -57,7 +57,7 @@ unset.
 <p>All <{iframe}> <a>elements</a> have an associated <dfn>iframe fullscreen flag</dfn>. Unless
 stated otherwise it is unset.
 
-<p>All <a for=/>documents</a> have an associated <dfn>fullscreen element</dfn>. The
+<p>All <a for=/>documents</a> have an associated <dfn export>fullscreen element</dfn>. The
 <a>fullscreen element</a> is the topmost <a>element</a> in the <a for=/>document</a>'s
 <a>top layer</a> whose <a>fullscreen flag</a> is set, if any, and null otherwise.