[css3-ui] Editorial post-bikeshed-fix: use abbr instead of invalid/obsolete acronym element

Author: Tantek Çelik

css-ui/Overview.html

@@ -746,11 +746,11 @@ <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="cont
    <div data-fill-with="warning"></div>
 
    <p class="copyright" data-fill-with="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> © 2015 <a href="http://www.w3.org/">
-     <acronym title="World Wide Web Consortium">W3C</acronym>
+     <abbr title="World Wide Web Consortium">W3C</abbr>
     </a><sup>®</sup> (<a href="http://www.csail.mit.edu/">
-     <acronym title="Massachusetts Institute of Technology">MIT</acronym>
+     <abbr title="Massachusetts Institute of Technology">MIT</abbr>
     </a>, <a href="http://www.ercim.eu/">
-     <acronym title="European Research Consortium for Informatics and Mathematics">ERCIM</acronym>
+     <abbr title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr>
     </a>, <a href="http://www.keio.ac.jp/">Keio</a>, <a href="http://ev.buaa.edu.cn/">Beihang</a>). W3C <a href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>, <a href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a> and <a href="http://www.w3.org/Consortium/Legal/copyright-documents">document use</a> rules apply.
 </p>