Introduction  : 
                       Hi, Here i will code you how to increase the font size of the Web page from Client side using JQuery. Here i use a div tag where Content is placed inside it, and two buttons texted as '+' and '-' both Used to increase and decrease the Font size respectively in their Client events.Here I use JQuery 1.7 google CDN, it also works in JQuery 1.4,

Logic  : 
             Here i have declared default size 13 on each '+' button Click i have increased the font size to +1 and in each '-' Click i have decreased Font size to -1,Soon After increasing or decreasing the font size i have applied the font size to the '<div>' which the content is placed.
 Example: $(".divtag").css("font-size", size);

Asp Code (with JQuery) : 

<head id="Head1" runat="server">
<style type="text/css">

<script type="text/javascript" src=> </script>
<script type="text/javascript">
$(document).ready(function () {
 var defaultSize = 13;
 var size = defaultSize + "px";
 $(".divtag").css("font-size", size);
 $('#ZoomIn').click(function () {
 defaultSize = defaultSize + 1;
 var size = defaultSize + "px";
 $(".divtag").css("font-size", size);
 return false;
 $('#ZoomOut').click(function () {
 defaultSize = defaultSize - 1;
 var size = defaultSize + "px";
 $(".divtag").css("font-size", size);
 return false;

<form id="form1" runat="server">
<div align="center">
<div class="divtag">
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<asp:Button ID="ZoomIn" runat="server" Text="+" Font-Bold="true" />
<asp:Button ID="ZoomOut" runat="server" Text="-" Font-Bold="true" />