Fix validation error on homepage: The center element is obsolete. Use CSS instead.

assets/css/styles.css

@@ -397,7 +397,10 @@ figcaption {
     display:inline;
 }
 
-#flattr {font-size:0.6em;}
+#flattr {
+    font-size:0.6em;
+    text-align:center;
+}
 
 #flattr a {
     margin-right: 1em;

content/_index.html

@@ -11,11 +11,11 @@ title: Secure messaging, anywhere
   </ul><p></p>
 
 <a href="https://play.google.com/store/apps/details?id=org.briarproject.briar.android" rel="noopener noreferrer" target="_blank">
-<img id="playbutton" src="/img/google_play_badge_web_generic.png" alt="Get it on Google Play" border="0"></a>
+<img id="playbutton" src="/img/google_play_badge_web_generic.png" alt="Get it on Google Play"></a>
 
 
 <a href="/fdroid">
-<img id="fdroidbutton" src="img/fdroid_badge.png" alt="Get it on F-Droid" border="0"></a>
+<img id="fdroidbutton" src="img/fdroid_badge.png" alt="Get it on F-Droid"></a>
 
 
 <p></p></div> <!-- intro --><p></p>
@@ -25,12 +25,10 @@ title: Secure messaging, anywhere
 </div>
 <div class="clearboth"></div>
 <div id="flattr">
-  <center>
   <a href="https://liberapay.com/Briar/donate" rel="noopener noreferrer" target="_blank"><img alt="Donate using Liberapay" src="img/liberapay.svg"></a>
-  <a href="https://flattr.com/t/592836/" rel="noopener noreferrer" target="_blank"><img src="img/flattr-badge-large.png" alt="Flattr this" title="Flattr this" border="0"></a>
+  <a href="https://flattr.com/t/592836/" rel="noopener noreferrer" target="_blank"><img src="img/flattr-badge-large.png" alt="Flattr this" title="Flattr this"></a>
   <br>
   Bitcoin: 1NZCKkUCtJV2U2Y9hDb9uq8S7ksFCFGR6K
-  </center>
 </div> <!-- flattr --><p></p>
 
       </div>