Can I change background color of paragraph when I click on a link in the page.
<a href="#p4">Go to P4</a> <p id="p4">I'm P4</p>
what I need write in css to get -
p { background-color: yellow; }
You can not change the attribute of an element by clicking on another element with CSS, but it can be with JavaScript. Just add <script> tag to end of your <body> and onclick() to <a>:
<body>
    <a href="#p4" onclick="changeColor()">Go to P4</a>
    <p id="p4">I'm P4</p>
    <script>
      function changeColor() {
      document.getElementById("p4").style.backgroundColor = "yellow";
      }
    </script>
</body>