Fix #526. Let ShadowRoot.styleSheets return an empty StyleSheetList i…

…f it is not in a shadow-including document

spec/shadow/index.html

@@ -1487,6 +1487,9 @@ <h3>Extensions to the <code><a href="https://dom.spec.whatwg.org/#documentorshad
           <li>
             Regarding <code>elementFromPoint</code> and <code>elementsFromPoints</code>, they should return the result of running the <a>retargeting algorithm</a> with <a>context object</a> and the original result as input.
           </li>
+          <li>
+            Regarding <code>styleSheets</code>, it should return an empty <a>StyleSheetList</a> if the context object is not <a>in a shadow-including document</a>.
+          </li>
           <li>
             We should remove these methods and attributes from Document when this section is upstreamed to DOM Standard.
           </li>