I think what you are referring to is called "gradient border".
I do believe this treats the border as an image instead of line rendering element.
Examples:
.top2btm {
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%;
}
This one is a corner-box border graidient:
.btm-lt { position: relative; }
.btm-lt:before, .bot-left:after {
content: "";
position: absolute;
bottom: -3px;
left: -3px;
}
.btm-lt:before {
top: -3px;
width: 3px;
background-image: -webkit-gradient(linear, 0 100%, 0 0, from(#000), to(transparent));
background-image: -webkit-linear-gradient(transparent, #000);
background-image: -moz-linear-gradient(transparent, #000);
background-image: -o-linear-gradient(transparent, #000);
}
.btm-lt:after {
right: -3px;
height: 3px;
background-image: -webkit-gradient(linear, 0 0, 100% 0, from(#000), to(transparent));
background-image: -webkit-linear-gradient(left, #000, transparent);
background-image: -moz-linear-gradient(left, #000, transparent);
background-image: -o-linear-gradient(left, #000, transparent); }