doctype打破css更改功能

I created a php file that generates a page as follows:

{session code}

<!doctype html>
<html>
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
    <title>Kenshoo Zabbix Console</title>
    <?php
    print $cWeb->getCss($page);
    print $cWeb->getScripts($page,$user);
    ?>

</head>
<body>
...
 <table id="alerts" class="tablesorter" style="width:100%" cellspacing="0" cellpadding="0">
 </table>
...
</body>
</html>

I created a range input that triggers the following function:

function adjustFont() {
    $(".tablesorter td").css("font-size", $("#sizeSelector").val());
    $(".tablesorter th").css("font-size", $("#sizeSelector").val());
}

Ever since I added the doctype tag, the adjustFont stopped working, removing the doctype returns the functionality but breaks the css.

Any idea what may be causing this issue?

Thanks!