html: fix handling of the end of <style> and <script>

Example:

<style>
p {
    border: "</style>";
    padding: "/>";
    margin: 5;
}
</style>

<script language="/>" />

<script language="javascript">
var a = "/>";
var b = "</script>";
var c = 5;
</script>
<p>
var d = 6;
</p>

https://bugzilla.gnome.org/show_bug.cgi?id=614072