<script>
window.onload = function () {//onload begin
  var x = 2;
  var y = 2;
  var mas = [];

  for (var i = 0; i < x; i++) {
     mas[i] = [];
    for (var j = 0; j < y; j++) {
      mas[i][j] = parseInt(prompt('Введите элемент [' + i + ',' + j + ']'));
      document.write('[' + i + ',' + j + '] = ' + mas[i][j] + '; ');
    }
    document.write('<br>')
  }

  var max = mas[0][0];
  for (var i = 0; i < x; i++) {
    for (var j = 0; j < y; j++) {
      if (mas[i][j] > max) {
        max = mas[i][j]; 
      }
    }
  }
  document.write('<br><br>max = ' + max)

}//onload end
</script>