What Does 'zoom' do in CSS?

I found that some jQuery Plugin, in their css rule uses 'zoom' descriptor, I even Look into w3c website and found that it is used to magnify but how can I actually implement it? Or I have to Define some viewport? And how do I define such viewport ? Or i am wrong about the whole stuff ?

is it possible to use it like

a {
    zoom:1;
}

a:hover {
   zoom:2;
}

Zoom is not included in the CSS specification, but it is supported in IE, Safari 4, Chrome (and you can get a somewhat similar effect in Firefox with -moz-transform: scale(x) since 3.5). See here.

So, all browsers

 zoom: 2;
 zoom: 200%;

will zoom your object in by 2, so it's like doubling the size. Which means if you have

a:hover {
   zoom: 2;
}

On hover, the <a> tag will zoom by 200%.

Like I say, in FireFox 3.5+ use -moz-transform: scale(x), it does much the same thing.

Edit: In response to the comment from thirtydot, I will say that scale() is not a complete replacement. It does not expand in line like zoom does, rather it will expand out of the box and over content, not forcing other content out of the way. See this in action here. Furthermore, it seems that zoom is not supported in Opera.

This post gives a useful insight into ways to work around incompatibilities with scale and workarounds for it using jQuery.

Surprised that nobody mentioned that zoom: 1; is useful for IE6-7, to solve most IE-only bugs by triggering hasLayout.

CSS zoom property is widely supported now > 86% of total browser population.

See: http://caniuse.com/#search=zoom

document.querySelector('#sel-jsz').style.zoom = 4;

#sel-001 {
  zoom: 2.5;
}
#sel-002 {
  zoom: 5;
}
#sel-003 {
  zoom: 300%;
}

<div id="sel-000">IMG - Default</div>

<div id="sel-001">IMG - 1X</div>

<div id="sel-002">IMG - 5X</div>

<div id="sel-003">IMG - 3X</div>


<div id="sel-jsz">JS Zoom - 4x</div>

This property controls the magnification level for the current element. The rendering effect for the element is that of a “zoom” function on a camera. Even though this property is not inherited, it still affects the rendering of child elements.

Example

 div { zoom: 200% }
 <div style=”zoom: 200%”>This is x2 text </div>

zoom is a css3 spec for the @viewport descriptor, as described here

http://dev.w3.org/csswg/css-device-adapt/#zoom-desc

used to zoom the entire viewport ('screen'). it also happens to zoom individuals elements in a lot of browsers, but not all. css3 specifies transform:scale should be used to achieve such an effect:

http://www.w3.org/TR/css3-transforms/#transform-functions

but it works a little different than the 'element zoom' in those browsers that support it.

Only IE and WebKit support zoom, and yes, in theory it does exactly what you're saying.

Try it out on an image to see it's full effect :)

As Joshua M pointed out, the zoom function isn't supported only in Firefox, but you can simply fix this as shown:

div.zoom {
  zoom: 2; /* all browsers */
 -moz-transform: scale(2);  /* Firefox */
}