你需要用js做，只有css做不了。

// Function to add span on last word
function spanLastWord(element)
{
    var pieces = element.innerHTML.split(" ");
    var last = pieces.pop();
    var first = pieces.join(" ");
    element.innerHTML = first + '<span id=\"lastWord\"> ' + last + '</span>';
}

// Function to position element
function positionElement(){
  // Get title
  var lastwordContainer =  document.getElementById("lastwordContainer");
  spanLastWord(lastwordContainer);

  // Get last word
  var lastWord =  document.getElementById("lastWord");

  // Get last word position
  var rect = lastWord.getBoundingClientRect();
  //console.log(rect.top, rect.right, rect.bottom, rect.left);

  // Get the div to position
  var divLastWord =  document.getElementById("divLastWord");
  divLastWord.style.top = rect.bottom - 10 + 'px';
  divLastWord.style.left = rect.right - 80 + 'px';
}
// event listener to position in cas of resize
window.addEventListener('resize', function(event) {

  positionElement();

}, true);

positionElement();

#divLastWord{
  border: 2px solid yellow;
  position: absolute;
}

<h4 id="lastwordContainer" class="">Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aenean ut odio arcu. Aenean augue massa, placerat fermentum consectetur sit amet, pulvinar nec mi. Mauris velit tellus, posuere nec cursus ut, mattis ut orci. Maecenas tincidunt tellus quis orci finibus tristique. Suspendisse tristique augue ultrices leo tristique ultrices. Vestibulum semper lacinia pulvinar. Pellentesque dignissim nunc in vulputate bibendum. Praesent risus nibh, suscipit ac blandit a, rhoncus vel sem. Cras interdum fermentum augue eget pellentesque. Integer velit massa, cursus in volutpat cursus, iaculis ac elit. In quis ex dui. Ut a tellus erat. Nam fermentum id elit eu tempus. Orci varius natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus. Cras a ultricies arcu, et hendrerit nunc. Pellentesque vitae dignissim arcu, ut aliquam nibh.</h4>
<div id="divLastWord">I am on the last word</div>