If you want to separate your triangle from the rectangle, consider using this approach:
<div id="rectangle"><div id="mask"></div></div>
#rectangle{width:300px; height:120px; position:relative; margin-top:100px; background: rgb(30,87,153);
background: -moz-linear-gradient(top, rgba(30,87,153,1) 0%, rgba(125,185,232,1) 100%);
background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,rgba(30,87,153,1)), color-stop(100%,rgba(125,185,232,1)));
background: linear-gradient(to bottom, rgba(30,87,153,1) 0%,rgba(125,185,232,1) 100%);
filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#1e5799', endColorstr='#7db9e8',GradientType=0 );}
#rectangle:after{content:" "; position:absolute; width:0px; height:0px; top:0; left:100px;border:1px solid white; border-color:transparent white transparent white; border-width:0px 50px 50px 50px;}
#mask{position:absolute; left:0; width:100px; height:50px; background:white;}
#mask:after{position:absolute; content:" "; left:200px; width:100px; background:white; height:50px; }
Explore the fiddle here.