j0hnik,
...
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css"/>
<style type="text/css">
.email-address:before {
content: "Email address: ";
}
</style>
<script>
// for styleSheets
var style = document.styleSheets;
for (var i=0; i<style.length; i++) {
//for cssRules
var rules = style[i].cssRules;
for (var j=0; j<rules.length; j++) {
if(rules[j].selectorText == ".email-address::before") {
alert(rules[j].style["content"]);
break;
}
}
}
</script>
</body>
</html>