How to draw a trapezium/trapezoid with css3?

You can use some CSS like this:

#trapezoid {
    border-bottom: 100px solid red;
    border-left: 50px solid transparent;
    border-right: 50px solid transparent;
    height: 0;
    width: 100px;
}
<div id="trapezoid"></div>

It is really cool to make all this shapes, Take a look to more nice shapes at:

http://css-tricks.com/examples/ShapesOfCSS/

EDIT: This css is applied to a DIV element


As this is quite old now, I feel it could use with some new updated answers with some new technologies.

CSS Transform Perspective

.trapezoid {
  width: 200px;
  height: 200px;
  background: red;
  transform: perspective(10px) rotateX(1deg);
  margin: 50px;
}
<div class="trapezoid"></div>

SVG

<svg viewBox="0 0 20 20" width="20%">
  <path d="M3,0 L17,0 L20,20 L0,20z" fill="red" />
</svg>

Canvas

var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.moveTo(30, 0);
ctx.lineTo(170, 0);
ctx.lineTo(200, 200);
ctx.lineTo(0, 200);
ctx.fillStyle = "#FF0000";
ctx.fill();
<canvas id="myCanvas" width="200" height="200"></canvas>