<html>
    <script>
window.onload = function () {
    setInterval(blink, 0);
}

var a = 0;

function blink() {
    document.getElementById("blink").style.backgroundColor = a ? "blue" : "green";
    a = !a;
}
    </script>
    <style>
#blink {
    width: 200px; height: 200px;
    display: inline-block;
}
    </style>
    <body>
        <div id="blink"></div>
    </body>
</html>