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>