madeas,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<style>
.block-1 {
box-shadow: 10px 20px 30px blue;
height: 100px;
}
</style>
<div class="block-1">123</div>
<script>
var style = document.styleSheets;
for (var i=0; i<style.length; i++) {
var rules = style[i].cssRules;
for (var k=0; k<rules.length; k++) {
if(rules[k].selectorText == ".block-1") {
var temp = rules[k].cssText.match(/\S+(?=:)/g)||[];
for (var n=0; n<temp.length; n++) {
var cs = temp[n];
if(cs == "box-shadow") rules[k].style[cs] += " inset"
}
}
}
}
</script>
</body>
</html>