Source of the built-in resource res:behavior-richtext-menu.htm - context menu of the <richtext> element.

<menu .richtext-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>
  <li>Paste special
      <menu>
        <li command="richtext:paste-plain-text">Paste text only</li>
	    <li command="richtext:paste-text-as-html">Paste text as HTML</li>
	    <li command="richtext:paste-html-no-tables">Paste without tables</li>
	  </menu>
  </li>
  <hr/>
  <li command="richtext:selectall"
	  >Select All<span class="accesskey">Ctrl+A</span></li>
  <div .cell-selection>
      <caption style="color:graytext">Cells:<img.hr/></caption>
	  <li command="richtext:merge-cells"
		  >Merge<span class="accesskey">Backspace</span></li>
	  <li command="richtext:split-cells-by-rows"
		  >Split by rows<span class="accesskey">Ctrl+1</span></li>
	  <li command="richtext:split-cells-by-cols"
		  >Split by columns<span class="accesskey">Ctrl+2</span></li>
  </div>
</menu>