рони, нужны проверки style может быть не один
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
.email-address:before{
content: "Email address: ";
}
</style>
</head>
<body>
<script>
[].forEach.call(document.styleSheets, el=> {
var a = [].find.call(el.rules, elt=>elt.selectorText == '.email-address::before');
a && alert(a.style.getPropertyValue("content"));
});
</script>
</body>
</html>