Here I'm using one ID(#mybox) and class(.box) but both are having the same declaration. moreover here working the background color only with ID. apart from the i know only difference between the ID and class but here as comparison with ID and class.it's working with ID what is difference.and in this case what is strongest in both.
<!DOCTYPE html>
<html>
<head>
<title>Page Title</title>
<style>
#mybox{background-color: red;}
.box{background-color:yellow;}
</style>
</head>
<body>
<div id="mybox" class="box">
boxes block
</div>
</body>
</html>