1. <body>
    2.       <div id="nav"></div>
    5. <script>
    6.     var nav=document.getElementById("nav")
    7.     window.onscroll=function(){
    9.         var scrollTop=window.scrollY;
    11.         if(scrollTop>200){
    12.             nav.style.opacity=1;
    13.         }
    14.         else
    15.         nav.style.opacity=scrollTop/200;
    16.     }
    17. </script>
    19. </body>