add: type button

Author: Gabriele-Zini

Diff for: index.html

@@ -16,9 +16,9 @@
         <div class="thumbs"></div>
       </div>
       <div class="buttons">
-        <button id="start">start autoplay</button>
-        <button id="invert">invert autoplay</button>
-        <button id="stop">stop autoplay</button>
+        <button id="start" type="button">start autoplay</button>
+        <button id="invert" type="button">invert autoplay</button>
+        <button id="stop" type="button">stop autoplay</button>
       </div>
     </div>
     <script src="./js/data.js"></script>