Fix the definition of the ol.start IDL attribute

Closes #1750, by settling on the Firefox/Edge behavior. There is a
separate issue of defining the "starting value" concept correctly; that
is #1911.

source

@@ -17567,9 +17567,17 @@ interface <dfn>HTMLOListElement</dfn> : <span>HTMLElement</span> {
 
   <div w-nodev>
 
-  <p>The <dfn><code data-x="dom-ol-reversed">reversed</code></dfn>, <dfn><code data-x="dom-ol-start">start</code></dfn>, and <dfn><code data-x="dom-ol-type">type</code></dfn> IDL attributes must <span>reflect</span> the
-  respective content attributes of the same name. The <code data-x="dom-ol-start">start</code> IDL
-  attribute has the same default as its content attribute.</p>
+  <p>The <dfn><code data-x="dom-ol-reversed">reversed</code></dfn> and
+  <dfn><code data-x="dom-ol-type">type</code></dfn> IDL attributes must <span>reflect</span> the
+  respective content attributes of the same name.</p>
+
+  <p>The <dfn><code data-x="dom-ol-start">start</code></dfn> IDL attribute must <span>reflect</span>
+  the content attribute of the same name, with a default value of 1.</p>
+
+  <p class="note">This means that the <code data-x="dom-ol-start">start</code> IDL attribute does
+  not necessarily match the list's <span data-x="concept-ol-start">starting value</span>, in cases
+  where the <code data-x="attr-ol-start">start</code> content attribute is omitted and the <code
+  data-x="attr-ol-reversed">reversed</code> content attribute is specified.</p>
 
   </div>