Source of the built-in resource res:behavior-text-menu.htm - context menu of <plaintext> and <textarea> elements.

<menu .plaintext-context>
  <li command="richtext:undo"
	  style="foreground-image:url(res:edit-undo.png)"
	  >Undo<span class="accesskey">Ctrl+Z</span></li>
  <hr/>
  <li command="richtext:cut" 
	  style="foreground-image:url(res:edit-cut.png)"
	  >Cut<span class="accesskey">Ctrl+X</span></li>
  <li command="richtext:copy"
	  style="foreground-image:url(res:edit-copy.png)"
	  >Copy<span class="accesskey">Ctrl+C</span></li>
  <li command="richtext:paste"
	  style="foreground-image:url(res:edit-paste.png)"
	  >Paste<span class="accesskey">Ctrl+V</span></li>
  <hr/>
  <li command="richtext:selectall"
	  >Select All<span class="accesskey">Ctrl+A</span></li>
</menu>