<style>
#clock{font-size:100px}
</style>

css:
  #clock{font-size:100px}

<script>
  var clk=document.getElementById('clock');
  var now=new Date();
  clk.innerHTML=now.toLocaleString();
</script>

javascript:
  var clk=document.getElementById('clock');
  var now=new Date();
  clk.innerHTML=now.toLocaleString();

<style>
#clock{font-size:100px}
</style>

  #clock{font-size:100px}