How to create fluid trapezoid image with css?

Updated Answer (Pure CSS3)

Extreme requirements sometimes need extreme solutions. I’ve built upon my original answer (below) to make a pure css solution that works (and can be made to work better, if you want to invest the time in it). The current example is a bit “choppy” in rendering, which may be okay for you, but if not, you will need to extend the already obscene number of @media queries that are driving it (it would likely be much easier to implement using LESS or SASS; I put together a formula driven excel spreadsheet to help rapidly generate the numbers). It uses this code:

HTML

<div class="box">
   <img src="https://stackoverflow.com/questions/9886337/yourImage.jpg" />
</div> 

CSS

.box{
    height:300px;  
    min-width: 100px; /*could be different, but you ought to have some min*/
    overflow:hidden;
}

.box img {
    height: 100%;
    width: 100%;   
    -ms-transform-origin: 100% 100%; /* IE 9 */
    -webkit-transform-origin: 100% 100%; /* Safari and Chrome */
    -moz-transform-origin: 100% 100%; /* Firefox */
    -o-transform-origin: 100% 100%; /* Opera */
    transform-origin: 100% 100%;
}

/*Sample... you need alot of these--my fiddle example has 51*/
@media screen and (min-width:  100px) { 
  .box { 
    -ms-transform:skewY(45deg); 
    -moz-transform:skewY(45deg); 
    -webkit-transform:skewY(45deg); 
    -o-transform:skewY(45deg); 
    transform:skewY(45deg); 
  } 
  .box img { 
    -ms-transform:skewY(-90deg); 
    -moz-transform:skewY(-90deg); 
    -webkit-transform:skewY(-90deg); 
    -o-transform:skewY(-90deg); 
    transform:skewY(-90deg);
  }
}

Here’s how to calculate the degrees

Assuming height: 300px with the narrow side approximately 100px tall and equal angles on the trapezoid. This means the upper and lower offset is (300px - 100px) / 2 = 100px. Then the .box angles are set off the @media query min-width amounts according to this formula:

Angle = arctan(100/min-width) /*100 is the upper/lower offset as above*/

For the .box img angle take the Angle and multiply by -2. So that will yield your .box and .box img media queries and transforms as this pseudocode:

@media screen and (min-width: [your target min-width]) { 
  .box {transform: skewY(Angle)}
  .box img {transform: skewY(-2*Angle)}
}

How smooth it functions depends completely upon how micro scale you make your changes to min-width to get a new angle setting. As I stated in my comment in the CSS code above, my example uses 51 media query calls and still has some choppiness to it.

Would it be better to use some javascript solution instead… probably, but that is totally up to the designer, so I offer this here as purely a proof of concept that it can be made to work with pure css.

Original Answer

This seems to be achieving a fluid width. I don’t know how much control you want of either how much or what part of the image is being shown, so it may not entirely fit your needs, but it does make a flexible width image using transforms to give it a fake perspective look.