How to zoom content of a web page?

How to zoom content of a开发者_JS百科 web page? Like for example on familyecho.com you can zoom in and zoom out. How do they do it?

1. Size everything zoomable with "em" units
2. Make your zoom Javascript set the "font-size" of the parent zoomable element up and down by percentages

Here's a simple goofy sample: http://gutfullofbeer.net/zoom.html

It uses jQuery but that's of course not necessary.

They are likely changing the css attributes for width/height and font size. It looked to me from my brief visit that it was only boxes and text that changed shape.

//jQuery...
$.(function(e) {
    $('#plusButton').click(function() {
        $('#stuffToGrow').css({
          width: function(index, value) {
              return value * 2;
          }, 
          height: function(index, value) {
             return value * 2;
          });
    });
});

You get the idea...