The solution is quite simple
Just get jQuery and jQuery ui
<head>
<link rel="stylesheet" href="http://code.jquery.com/ui/1.10.2/themes/smoothness/jquery-ui.css"/>
<script src="http://code.jquery.com/jquery-1.9.1.js"></script>
<script src="http://code.jquery.com/ui/1.10.2/jquery-ui.js"></script>
</head>
Create the elements
<div id="slider-range-max"></div>
<p id="text">Text</p>
and change font-size on slide event.
$( "#slider-range-max" ).slider({
range: "max",
min: 10,
max: 70,
value: 10,
slide: function( event, ui ) {
$( "#text" ).css( { fontSize: ui.value} );
}
});