Хотя можно попробовать что-нибудь по экспиерементировать с градентами:
border-width: 0;
border-left-width: 10px;
border-image: -moz-linear-gradient(top, rgba(52,152,219,1) 0%, rgba(255,255,255,0) 100%);
border-image: -webkit-gradient(linear, left top, left bottom, color-stop(0%,rgba(52,152,219,1)), color-stop(100%,rgba(255,255,255,0)));
border-image: -webkit-linear-gradient(top, rgba(52,152,219,1) 0%,rgba(255,255,255,0) 100%);
border-image: -o-linear-gradient(top, rgba(52,152,219,1) 0%,rgba(255,255,255,0) 100%);
border-image: -ms-linear-gradient(top, rgba(52,152,219,1) 0%,rgba(255,255,255,0) 100%);
border-image: linear-gradient(to bottom, rgba(52,152,219,1) 0%,rgba(255,255,255,0) 100%);
border-image-slice: 1;