Use CSS3 transitions with gradient backgrounds

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>  

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 Chromium based browsers using the @property workaround (but is not working in Firefox). Please see @mahozad's answer below.

A solution is to use background-position to mimic the gradient transition. This solution was used in Twitter Bootstrap a few months ago.

Update

http://codersblock.blogspot.fr/2013/12/gradient-animation-trick.html?showComment=1390287622614

Here is a quick example:

Link state

 .btn {
  font-family: "Helvetica Neue", Arial, sans-serif;
  font-size: 12px;
  font-weight: 300;
  position: relative;
  display: inline-block;
  text-decoration: none;
  color: #fff;
  padding: 20px 40px;
  background-image: -moz-linear-gradient(top, #50abdf, #1f78aa);
  background-image: -webkit-gradient(linear, 0 0, 0 100%, from(#50abdf), to(#1f78aa));
  background-image: -webkit-linear-gradient(top, #50abdf, #1f78aa);
  background-image: -o-linear-gradient(top, #50abdf, #1f78aa);
  background-image: linear-gradient(to bottom, #50abdf, #1f78aa);
  background-repeat: repeat-x;
  filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#ff50abdf', endColorstr='#ff1f78aa', GradientType=0);
  background-repeat: repeat-y;
  background-size: 100% 90px;
  background-position: 0 -30px;
  -webkit-transition: all 0.2s linear;
     -moz-transition: all 0.2s linear;
       -o-transition: all 0.2s linear;
          transition: all 0.2s linear;
}

Hover state

.btn:hover {
   background-position: 0 0;
}