<p>This is the content of the page</p>
<div id="modal" class="modal">
  <div class="warning">
    <div>
      <span class="exit">&times;</span>
      <a href="https://www.google.com">Google</a>
    </div>
    <a href="https://www.google.com/random">Some SharePoint URL</a>
  </div>
</div>

.modal {
  display: block;
  position: fixed;
  left: 0;
  top: 0;
  padding-top: 20px;
  z-index: 1;
  width: 100%;
  height: 100%;
  background-color: grey;
  background-color: rgba(0, 0, 0, 0.5);
}

.warning {
  background-color: #fefefe;
  margin: auto;
  padding: 20px;
  border: 1px solid #888;
  width: 80%;
}

.exit {
  color: #aaaaaa;
  float: right;
  font-size: 28px;
  font-weight: bold;
  cursor: pointer;
}

var modal = document.getElementById('modal');
var span = document.getElementsByClassName("exit")[0];

span.onclick = function() {
  modal.style.display = "none";
}