JSFiddle - React, Tailwind, and code Playground

HTML

<div id="borderbox">
    <div id="box">
         
    </div>
</div>

CSS

#borderbox {
    background-color: #eee; /* fallback color if gradients are not supported */
    background-image: -webkit-linear-gradient(to right, #000, #fff); /* For Chrome and Safari */
    background-image:    -moz-linear-gradient(to right, #000, #fff); /* For old Fx (3.6 to 15) */
    background-image:     -ms-linear-gradient(to right, #000, #fff); /* For pre-releases of IE 10*/
    background-image:      -o-linear-gradient(to right, #000, #fff); /* For old Opera (11.1 to 12.0) */
    background-image:         linear-gradient(to right, #000, #fff); /* Standard syntax; must be last */    
    width: 500px;
    height: 200px;
    display: block;
    padding: 1px 0 0 0;
    opacity: 0.5;
    border-top: 1px dashed #ccc;
}
#box { background: #fff; width: 500px; height: 200px;  margin-top: -1px; }