Edit in JSFiddle

<label for="txtFirstName" class="required">First Name</label>
<br />
<input id="txtFirstName" type="text" required />
/* Css Selector for tag of type label with class required */
label.required 
{
    font-weight: bold;
    color: orange;
}

/* Append text */
label.required::after
{
    content: "*"
}