[resize-observer] Update initial value of lastReportedSizes

Author: TremayneChrist

resize-observer-1/Overview.bs

@@ -74,6 +74,8 @@ counterpart to {{Window}}'s {{Window/resize}} event.
 
 ResizeObserver's notifications can be used to respond to changes in {{Element}}'s size. Some interesting facts about these observations:
 
+* Observation will fire when observation starts.
+
 * Observation will fire when watched Element is inserted/removed from DOM.
 
 * Observation will fire when watched Element <a>display</a> gets set to none.
@@ -82,8 +84,6 @@ ResizeObserver's notifications can be used to respond to changes in {{Element}}'
 
 * Observations will not be triggered by CSS transforms.
 
-* Observation will fire when observation starts if Element is [being rendered](https://html.spec.whatwg.org/#being-rendered), and Element's size is not 0,0.
-
 <div class="example">
   <pre highlight="html">
     &lt;canvas id="ellipse" style="display:block">&lt;/canvas>
@@ -333,7 +333,7 @@ interface ResizeObservation {
 
         3. Set |this| internal {{ResizeObservation/observedBox}} slot to |observedBox|.
 
-        4. Set |this| internal {{ResizeObservation/lastReportedSizes}} slot to [(0,0)].
+        4. Set |this| internal {{ResizeObservation/lastReportedSizes}} slot to [(-1,-1)].
 
     : <dfn method lt="isActive()">isActive()</dfn>
     ::