allasan,
<!DOCTYPE HTML>
<html>
<head>
<style type="text/css">
p {
font-size: 200%;
}
</style>
</head>
<body>
<span class="fon" onclick="alert(getRules('p')['font-size'])">click me</span>
<script>
function getRules(k) {
for (var e = {}, f = document.styleSheets, c = 0; c < f.length; c++)
for (var a = f[c].cssRules, b = 0; b < a.length; b++)
if (a[b].selectorText == k)
for (var g = a[b].cssText.match(/[-\w]+(?=:)/g) || [], d = 0; d < g.length; d++) {
var h = g[d],
l = h.replace(/-(\w)/g, function(b, a) {
return a.toUpperCase()
});
e[h] = a[b].style[l] }
return e
};
</script>
</body>
</html>