Make <tr>'s deleteCell() and <tbody>'s deleteRow() respect -1

Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=29014
Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=29015

source

@@ -36911,13 +36911,19 @@ the cell that corresponds to the values of the two dice.
   <code>tr</code> element in the <code data-x="dom-tbody-rows">rows</code> collection, and finally
   must return the newly created <code>tr</code> element.</p>
 
-  <p>The <dfn><code data-x="dom-tbody-deleteRow">deleteRow(<var>index</var>)</code></dfn>
-  method must remove the <var>index</var>th element in the <code
-  data-x="dom-tbody-rows">rows</code> collection from its parent. If <var>index</var> is
-  less than zero or greater than or equal to the number of elements in the <code
-  data-x="dom-tbody-rows">rows</code> collection, the method must instead throw an
+  <p>The <dfn><code data-x="dom-tbody-deleteRow">deleteRow(<var>index</var>)</code></dfn> method
+  must, when invoked, act as follows:
+
+  <p>If <var>index</var> is less than &#x2212;1 or greater than the number of elements in
+  the <code data-x="dom-tbody-rows">rows</code> collection, the method must throw an
   <code>IndexSizeError</code> exception.</p>
 
+  <p>If <var>index</var> is &#x2212;1, remove the last element in the <code
+  data-x="dom-tbody-rows">rows</code> collection from its parent.</p>
+
+  <p>Otherwise, remove the <var>index</var>th element in the <code
+  data-x="dom-tbody-rows">rows</code> collection from its parent.</p>
+
   </div>
 <!--TOPIC:HTML-->
 
@@ -37171,12 +37177,18 @@ the cell that corresponds to the values of the two dice.
   must return the newly created <code>td</code> element.</p>
 
   <p>The <dfn><code data-x="dom-tr-deleteCell">deleteCell(<var>index</var>)</code></dfn>
-  method must remove the <var>index</var>th element in the <code
-  data-x="dom-tr-cells">cells</code> collection from its parent. If <var>index</var> is less
-  than zero or greater than or equal to the number of elements in the <code
-  data-x="dom-tr-cells">cells</code> collection, the method must instead throw an
+  method must act as follows:</p>
+
+  <p>If <var>index</var> is less than &#x2212;1 or greater than the number of elements in
+  the <code data-x="dom-tr-cells">cells</code> collection, the method must throw an
   <code>IndexSizeError</code> exception.</p>
 
+  <p>If <var>index</var> is &#x2212;1, remove the last element in the <code
+  data-x="dom-tr-cells">cells</code> collection from its parent.</p>
+
+  <p>Otherwise, remove the <var>index</var>th element in the <code
+  data-x="dom-tr-cells">cells</code> collection from its parent.</p>
+
   </div>
 <!--TOPIC:HTML-->
 
@@ -116631,6 +116643,7 @@ INSERT INTERFACES HERE
   David Remahl,
   David Smith,
   David Woolley,
+  David Zbarsky,
   DeWitt Clinton,
   Dean Edridge,
   Dean Edwards,