window.addEvent('domready', function(){
    
    $('term-2')
        .getAllPreviousUntil('dt')
        .setStyle('background-color','#ecc');
   
    $('term-2')
        .getAllNextUntil('dt')
        .setStyle('background-color','#cce');
    
    document.getElement('li.item-a').getParentsUntil('.level-1')
      .setStyle('background-color', '#cec');
    
});

<dl>
  <dt>term 1</dt>
  <dd>definition 1-a</dd>
  <dd>definition 1-b</dd>
  <dd>definition 1-c</dd>
  <dd>definition 1-d</dd>

  <dt id="term-2">term 2</dt>
  <dd>definition 2-a</dd>
  <dd>definition 2-b</dd>
  <dd>definition 2-c</dd>

  <dt>term 3</dt>
  <dd>definition 3-a</dd>
  <dd>definition 3-b</dd>
</dl>

<ul class="level-1">
  <li class="item-i">I</li>
  <li class="item-ii">II
    <ul class="level-2">
      <li class="item-a">A</li>
      <li class="item-b">B
        <ul class="level-3">
          <li class="item-1">1</li>
          <li class="item-2">2</li>
          <li class="item-3">3</li>
        </ul>
      </li>
      <li class="item-c">C</li>
    </ul>
  </li>
  <li class="item-iii">III</li>
</ul>

MooShell