Specify the behaviour when the scrollSource's layout box is not a scr…

…oll container. Fixes issue #4.

Overview.bs

@@ -498,8 +498,8 @@ The <dfn>effective time range</dfn> of a {{ScrollTimeline}} is calculated as fol
 The [=current time=] of a {{ScrollTimeline}} is calculated
 as follows:
 
-1.  If {{scrollSource}} does not currently have a [=CSS layout box=], return an unresolved
-    time value.
+1.  If {{scrollSource}} does not currently have a [=CSS layout box=], or if its layout box is not
+    a [=scroll container=], return an unresolved time value.
 
 2.  Otherwise, let <var>current scroll offset</var> be the current scroll offset of {{scrollSource}}
     in the direction specified by {{orientation}}.

index.html

@@ -701,8 +701,8 @@ <h4 class="heading settled" data-level="3.1.4" id="current-time-algorithm"><span
 as follows:</p>
     <ol>
      <li data-md="">
-      <p>If <code class="idl"><a data-link-type="idl" href="#dom-scrolltimeline-scrollsource" id="ref-for-dom-scrolltimeline-scrollsource-10">scrollSource</a></code> does not currently have a <a data-link-type="dfn" href="https://www.w3.org/TR/cssom-view-1/#css-layout-box">CSS layout box</a>, return an unresolved
-time value.</p>
+      <p>If <code class="idl"><a data-link-type="idl" href="#dom-scrolltimeline-scrollsource" id="ref-for-dom-scrolltimeline-scrollsource-10">scrollSource</a></code> does not currently have a <a data-link-type="dfn" href="https://www.w3.org/TR/cssom-view-1/#css-layout-box">CSS layout box</a>, or if its layout box is not
+a <a data-link-type="dfn" href="https://drafts.csswg.org/css-overflow-3/#scroll-container">scroll container</a>, return an unresolved time value.</p>
      <li data-md="">
       <p>Otherwise, let <var>current scroll offset</var> be the current scroll offset of <code class="idl"><a data-link-type="idl" href="#dom-scrolltimeline-scrollsource" id="ref-for-dom-scrolltimeline-scrollsource-11">scrollSource</a></code> in the direction specified by <code class="idl"><a data-link-type="idl" href="#dom-scrolltimeline-orientation" id="ref-for-dom-scrolltimeline-orientation-10">orientation</a></code>.</p>
      <li data-md="">