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$('fruit').getElement('li:contains(Apple)')
    .getAllNextUntil(':contains(Pear)')
    .setStyle('color', '#e00');

<ul id="fruit">  
  <li>Apple</li>  
  <li>Banana</li>  
  <li>Grape</li>  

  <li>Strawberry</li>  
  <li>Pear</li>  
  <li>Peach</li>  
</ul>