Use CSS3 transitions with gradient backgrounds

I'm trying to transition on hover with css over a thumbnail so that on hover, the background gradient fades in. The transition isn't working, but if I simply change it to an rgba() value, it works fine. Are gradients not supported? I tried using an image too, it won't transition the image either.

I know it's possible, as in another post someone did it, but I can't figure out how exactly. Any help> Here's some CSS to work with:

#container div a {
  -webkit-transition: background 0.2s linear;
  -moz-transition: background 0.2s linear;
  -o-transition: background 0.2s linear;
  transition: background 0.2s linear;
  position: absolute;
  width: 200px;
  height: 150px;
  border: 1px #000 solid;
  margin: 30px;
  z-index: 2
}

#container div a:hover {
  background: -webkit-gradient(radial, 100 75, 100, 100 75, 0, from(rgba(0, 0, 0, .7)), to(rgba(0, 0, 0, .4)))
}

Gradients don't support transitions yet (although the current spec says they should support like gradient to like gradient transitions via interpolation.).

If you want a fade-in effect with a background gradient, you have to set an opacity on a container element and 'transition` the opacity.

(There have been some browser releases that supported transitions on gradients (e.g IE10. I tested gradient transitions in 2016 in IE and they seemed to work at the time, but my test code no longer works.)

Update: October 2018 Gradient transitions with un-prefixed new syntax [e.g. radial-gradient(...)]now confirmed to work (again?) on Microsoft Edge 17.17134. I don't know when this was added. Still not working on latest Firefox & Chrome / Windows 10.

Update: December 2021 This is now possible in recent browsers using the @property workaround - please see @mahozad's answer below.

One work-around is to transition the background position to give the effect that the gradient is changing: http://sapphion.com/2011/10/css3-gradient-transition-with-background-position/

CSS3 gradient transition with background-position

Although you can’t directly animate gradients using the CSS transition property, it is possible to animate the background-position property to achieve a simple gradient animation:

The code for this is dead simple:

#DemoGradient{  
    background: -webkit-linear-gradient(#C7D3DC,#5B798E);  
    background: -moz-linear-gradient(#C7D3DC,#5B798E);  
    background: -o-linear-gradient(#C7D3DC,#5B798E);  
    background: linear-gradient(#C7D3DC,#5B798E);  
  
    -webkit-transition: background 1s ease-out;  
    -moz-transition: background 1s ease-out;  
    -o-transition: background 1s ease-out;  
    transition: background 1s ease-out;  
  
    background-size:1px 200px;  
    border-radius: 10px;  
    border: 1px solid #839DB0;  
    cursor:pointer;  
    width: 150px;  
    height: 100px;  
}  
#DemoGradient:Hover{  
    background-position:100px;  
}  

<div id="DemoGradient"></div>