.gradient_border{
border-width: 3px;
border-style: solid;
-webkit-border-image: -webkit-gradient(linear, 0 0, 0 100%, from(black), to(rgba(0, 0, 0, 0))) 1 100%;
-webkit-border-image: -webkit-linear-gradient(black, rgba(0, 0, 0, 0)) 1 100%;
-moz-border-image: -moz-linear-gradient(black, rgba(0, 0, 0, 0)) 1 100%;
-o-border-image: -o-linear-gradient(black, rgba(0, 0, 0, 0)) 1 100%;
border-image: linear-gradient(to bottom, black, rgba(0, 0, 0, 0)) 1 100%;
}
Example:
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Proin semper maximus mattis. Sed eu ex molestie, interdum lorem ac, rhoncus arcu. Donec varius urna enim, malesuada gravida ligula volutpat in. Nam sit amet efficitur lacus. Fusce a condimentum ante.