<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
</head>
<body>
<script type="text/javascript">
function css_add(window_url) {
var tag_css = document.createElement('link');
tag_css.type = 'text/css';
tag_css.rel = 'stylesheet';
tag_css.href = window_url+'?css=1';
alert(window_url);
var tag_head = document.getElementsByTagName('head');
tag_head[0].appendChild(tag_css);
}
</script>
<h1 onclick="css_add('style.css');">Привет</h1>
</body>
</html>
1) createElement не style, а link
2) tag_css.rel = 'stylesheet'