element positioning proposal

  ul.menu, ul.hmenu {
   margin: 0px;
   padding: 0px;
   border: 1px solid red;
   position: absolute;
   }

  ul.menu li {
   list-style-type: none;
   border: 1px solid green;
   position: relative;
   }
  ul.hmenu li {
   display: inline;
   list-style-type: none;
   border: 1px solid green;
   }


<ul class="menu">
  <li>1
  <li>2 
  <li>3 
    <ul class="hmenu" style="left: 10px; bottom: -height +2px;">
      <li>a
      <li>b
    </ul>
   </li>
<ul>

NOTICE: bottom: -height +2px

it is equivalent to:
top: ownerElementHeight -2px