Editorial: clean up the definition of :default

Closes #4598.

Author: domenic

source

@@ -67573,12 +67573,8 @@ Demos:
     element falling into one of the following categories:</p>
 
     <ul>
-     <li><code>button</code> elements that are their form's <span>default button</span></li>
-
-     <li><code>input</code> elements whose <code data-x="attr-input-type">type</code> attribute is in
-     the <span data-x="attr-input-type-submit">Submit Button</span> or <span
-     data-x="attr-input-type-image">Image Button</span> state, and that are their form's
-     <span>default button</span></li>
+     <li><span data-x="concept-submit-button">Submit buttons</span> that are <span data-x="default
+     button">default buttons</span> of their <span>form owner</span>.</li>
 
      <li><code>input</code> elements to which the <code data-x="attr-input-checked">checked</code>
      attribute applies and that have a <code data-x="attr-input-checked">checked</code>