<html>
<body>
<style>
.yellow {color:yellow;}
.notYellow {color:blue;}
</style>
<script>
function togglecolor(){
$(function(){
var para = document.getElementById("para");
if(para.className != "yellow")
para.className = "yellow";
else para.className = "notYellow";
});
}
</script>
</head>
<body>
<h1 id="para" class="notYellow">Hello World! </h1>
<button onClick="togglecolor();">Press Me</button>
</body>
</html>