<body runat="server" id="Body" style="zoom:100%">
</body>
function zoomIn()
{
var Page = document.getElementById('Body');
var zoom = parseInt(Page.style.zoom) + 10 +'%'
Page.style.zoom = zoom;
return false;
}
function zoomOut()
{
var Page = document.getElementById('Body');
var zoom = parseInt(Page.style.zoom) - 10 +'%'
Page.style.zoom = zoom;
return false;
}
Now call above function in your Button click.