animation - CSS-moving text from left to right

Question

I want to create an animated HTML "marquee" that scrolls back and forth on a website:

<div class="marquee">This is a marquee!</div>

and the CSS:

.marquee {
    position: absolute;
    white-space: nowrap;
    -webkit-animation: rightThenLeft 4s linear;
}

@-webkit-keyframes rightThenLeft {
    0%   {left: 0%;}
    50%  {left: 100%;}
    100% {left: 0%;}
}

The problem is, the marquee doesn't stop when it reaches the right-hand edge of the screen; it moves all the way off the screen (making a horizontal scroll bar appear, briefly) and then comes back.

So, how do I make the marquee stop when its right-hand edge reaches the right-hand edge of the screen?

EDIT: Oddly, this does not work:

50% {right: 0%}

Answer 1

Somehow I got it to work by using margin-right, and setting it to move from right to left. http://jsfiddle.net/gXdMc/

Don't know why for this case, margin-right 100% doesn't go off the screen. :D (tested on chrome 18)

EDIT: now left to right works too http://jsfiddle.net/6LhvL/