如何用javascript调用外部css文件?

html
<script>
function changecss(){
var css=document.createElement('link');
css.href='c.css';
css.rel="stylesheet";
css.type="text/css";
document.body.appendChild(css);
}
window.onload=function(){
document.getElementById('changecss').onclick=changecss;
}
</script>
<span id="changecss">点</a>

css(c.css)
body{
background:red;
}

<script>
document.write('<link href="/iknow.css" rel="stylesheet" type="text/css">');
</script>

???????????? 这么个调用法？