JavaScript + CSS based Update Check

check_for_new_version.html

<script>
  var head = document.getElementsByTagName("head")[0];         
  var css = document.createElement('link');
  css.type = 'text/css';
  css.rel = 'stylesheet';
  css.href = 'http://product.website.here/update_1.0.css?'+Date.now();
//  css.href = 'http://product.website.here/update_1.2.css?'+Date.now(); Uncomment for version 1.2
  css.media = 'screen';
  head.appendChild(css)
</script>
<div id="update">
  PRODUCT_NAME <span id="new_version"></span> is now available (you have <span style="color:#fff">1.2</span>) 
  <a href="http://product.website.here/download.php">Download</a>
</div>

update_1.0.css

#update {
  display: block !important;
}

span#new_version:before {
	content: "1.2"; /* Current version is 1.2 */
	color: white;
}

update_1.2.css

#update {
  display: none !important;
}

span#new_version:before {
  content: "1.2"; /* Current version is 1.2 */
	color: white;
}