diff --git a/source b/source
index 0443fe393ab..f1fcd9f9234 100644
--- a/source
+++ b/source
@@ -50877,8 +50877,14 @@ interface <dfn interface>HTMLInputElement</dfn> : <span>HTMLElement</span> {
    <li><p>If this element's <code data-x="attr-input-type">type</code> attribute is in the <span
    data-x="attr-input-type-radio">Radio Button state</span>, then get a reference to the element in
    this element's <span>radio button group</span> that has its <span
-   data-x="concept-fe-checked">checkedness</span> set to true, if any, and then set this element's
-   <span data-x="concept-fe-checked">checkedness</span> to true.</p></li>
+   data-x="concept-fe-checked">checkedness</span> set to true, if any. If this element's <span
+   data-x="concept-fe-checked">checkedness</span> is true, then let <var>default button</var> be the
+   last element in this element's <span>radio button group</span>, in <span>tree order</span>, that
+   has a <code data-x="attr-input-checked">checked</code> content attribute, if any. If
+   <var>default button</var> is not null, then set <var>default button</var>'s <span
+   data-x="concept-fe-checked">checkedness</span> to true; otherwise, set this element's <span
+   data-x="concept-fe-checked">checkedness</span> to false. Otherwise, set this element's <span
+   data-x="concept-fe-checked">checkedness</span> to true.</p></li>
   </ol>
   </div>