Ich habs mal so probiert:
hr {
height: 1px;
margin: 30px 0;
background: -webkit-gradient(linear, 0 0, 100% 0, from(rgba(0,0,0,0)), color-stop(0.5, #333333), to(rgba(0,0,0,0)));
background: -webkit-linear-gradient(left, rgba(0,0,0,0), #333333, rgba(0,0,0,0));
background...