I'm trying to use the javascript function focus(), but it does not work on safari mobile. How do I fix this problem?
<html lang="en">
   <head>
      <meta charset="UTF-8">
      <title>demo</title>
      <script type="text/javascript">
         window.onload=function() {
           document.getElementById('username').focus();
         }
      </script>
   </head>
   <body>
     <input type="text" id="username" >
     <input type="text" >
   </body>
</html>