Make <li> ordinal value calculation precise

Fixes whatwg#1608. As noted there, the existing prose was using confusing
concepts of attributes being "treated as if they were absent" and
"attribute values" that were distinct from the usual meaning.

source

@@ -17487,27 +17487,32 @@ interface <dfn>HTMLOListElement</dfn> : <span>HTMLElement</span> {
   the attribute is omitted, the list is an ascending list (1, 2, 3, ...).</p>
 
   <p>The <dfn><code data-x="attr-ol-start">start</code></dfn> attribute, if present, must be a
-  <span>valid integer</span> giving the <span>ordinal value</span> of the first list item.</p>
+  <span>valid integer</span>. It is used to determine the <span data-x="concept-ol-start">starting
+  value</span> of the list.</p>
 
   <div w-nodev>
 
-  <p>If the <code data-x="attr-ol-start">start</code> attribute is present, user agents must <span
-  data-x="rules for parsing integers">parse it as an integer</span>, in order to determine the
-  attribute's value. The default value, used if the attribute is missing or if the value cannot be
-  converted to a number according to the referenced algorithm, is 1 if the element has no <code
-  data-x="attr-ol-reversed">reversed</code> attribute, and is the number of child <code>li</code>
-  elements otherwise.</p>
+  <p>An <code>ol</code> element has a <dfn data-x="concept-ol-start">starting value</dfn>, which is
+  an integer determined as follows:</p>
 
-  <p>The first item in the list has the <span>ordinal value</span> given by the <code>ol</code>
-  element's <code data-x="attr-ol-start">start</code> attribute, unless that <code>li</code> element
-  has a <code data-x="attr-li-value">value</code> attribute with a value that can be successfully
-  parsed, in which case it has the <span>ordinal value</span> given by that <code
-  data-x="attr-li-value">value</code> attribute.</p>
+  <ol>
+   <li>
+    <p>If the <code>ol</code> element has a <code data-x="attr-ol-start">start</code> attribute,
+    then:</p>
 
-  <p>Each subsequent item in the list has the <span>ordinal value</span> given by its <code
-  data-x="attr-li-value">value</code> attribute, if it has one, or, if it doesn't, the <span>ordinal
-  value</span> of the previous item, plus one if the <code data-x="attr-ol-reversed">reversed</code>
-  is absent, or minus one if it is present.</p>
+    <ol>
+     <li><p>Let <var>parsed</var> be the result of <span data-x="rules for parsing integers">parsing
+     the value of the attribute as an integer</span>.</p></li>
+
+     <li><p>If <var>parsed</var> is not an error, then return <var>parsed</var>.</p></li>
+    </ol>
+   </li>
+
+   <li><p>If the <code>ol</code> element has a <code data-x="attr-ol-reversed">reversed</code>
+   attribute, then return the number of child <code>li</code> elements.</p></li>
+
+   <li><p>Return 1.</p></li>
+  </ol>
 
   </div>
 
@@ -17699,26 +17704,60 @@ interface <dfn>HTMLLIElement</dfn> : <span>HTMLElement</span> {
   parent element's list, as defined for those elements. Otherwise, the list item has no defined
   list-related relationship to any other <code>li</code> element.</p>
 
-  <p>If the parent element is an <code>ol</code> element, then the <code>li</code> element has an
-  <dfn>ordinal value</dfn>.</p>
-
   <p>The <dfn><code data-x="attr-li-value">value</code></dfn> attribute, if present, must be a
-  <span>valid integer</span> giving the <span>ordinal value</span> of the list item.</p>
+  <span>valid integer</span>. It is used to determine the <span>ordinal value</span> of the list
+  item, when the parent element is an <code>ol</code> element.</p>
 
   <div w-nodev>
 
-  <p>If the <code data-x="attr-li-value">value</code> attribute is present, user agents must <span
-  data-x="rules for parsing integers">parse it as an integer</span>, in order to determine the
-  attribute's value. If the attribute's value cannot be converted to a number, the attribute must be
-  treated as if it was absent. The attribute has no default value.</p>
+  <p>If the parent element is an <code>ol</code> element, then the <code>li</code> element has an
+  <dfn>ordinal value</dfn>, which is an integer determined as follows:</p>
 
-  <p>The <code data-x="attr-li-value">value</code> attribute is processed relative to the element's
-  parent <code>ol</code> element (q.v.), if there is one. If there is not, the attribute has no
-  effect.</p>
+  <ol>
+   <li>
+    <p>If the <code>li</code> element has a <code data-x="attr-li-value">value</code> attribute,
+    then:</p>
+
+    <ol>
+     <li><p>Let <var>parsed</var> be the result of <span data-x="rules for parsing integers">parsing
+     the value of the attribute as an integer</span>.</p></li>
+
+     <li><p>If <var>parsed</var> is not an error, then return <var>parsed</var>.</p></li>
+    </ol>
+   </li>
+
+   <li><p>If the <code>li</code> element is the first <code>li</code> child of its parent
+   <code>ol</code> element, then return the <code>ol</code> element's <span
+   data-x="concept-ol-start">starting value</span>.</p></li>
+
+   <li><p>Let <var>previous</var> be the <span>ordinal value</span> of the first preceding sibling
+   that is an <code>li</code> element.</p></li>
+
+   <li><p>If the parent <code>ol</code> element has a <code
+   data-x="attr-ol-reversed">reversed</code> attribute, then return <var>previous</var> &#x2212;
+   1.</p></li>
+
+   <li><p>Return <var>previous</var> + 1.</p></li>
+  </ol>
 
   <p>The <dfn><code data-x="dom-li-value">value</code></dfn> IDL attribute must <span>reflect</span>
   the value of the <code data-x="attr-li-value">value</code> content attribute.</p>
 
+  <div class="example">
+   <p>The element's <code data-x="dom-li-value">value</code> IDL attribute does not directly
+   correspond to its <span>ordinal value</span>; it simply <span data-x="reflect">reflects</span>
+   the content attribute. For example, given this list:
+
+   <pre>&lt;ol>
+ &lt;li>Item 1
+ &lt;li value="3">Item 3
+ &lt;li>Item 4
+&lt;/ol></pre>
+
+   <p>The <span data-x="ordinal value">ordinal values</span> are 1, 3, and 4, whereas the <code
+   data-x="dom-li-value">value</code> IDL attributes return 0, 3, 0 on getting.</p>
+  </div>
+
   </div>
 
   <div class="example">
@@ -116729,7 +116768,7 @@ interface <dfn>External</dfn> {
     <tr>
      <th> <code data-x="">start</code>
      <td> <code data-x="attr-ol-start">ol</code>
-     <td> <span>Ordinal value</span> of the first item
+     <td> <span data-x="concept-ol-start">Starting value</span> of the list
      <td> <span>Valid integer</span>
     <tr>
      <th> <code data-x="">step</code>
@@ -119089,6 +119128,7 @@ INSERT INTERFACES HERE
   David Vest,
   David Woolley,
   David Zbarsky,
+  Dave Methvin,
   DeWitt Clinton,
   Dean Edridge,
   Dean Edwards,