JSFiddle - React, Tailwind, and code Playground

HTML

<div class="main">
    <div class="corner"><span></span></div>
</div>

CSS

.main {
    background: -moz-linear-gradient(top, #ffffff 0%, #e8e8e8 100%); /* FF3.6+ */
    background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,#ffffff), color-stop(100%,#e8e8e8)); /* Chrome,Safari4+ */
    background: -webkit-linear-gradient(top, #ffffff 0%,#e8e8e8 100%); /* Chrome10+,Safari5.1+ */
    background: -o-linear-gradient(top, #ffffff 0%,#e8e8e8 100%); /* Opera 11.10+ */
    background: -ms-linear-gradient(top, #ffffff 0%,#e8e8e8 100%); /* IE10+ */
    background: linear-gradient(to bottom, #ffffff 0%,#e8e8e8 100%); /* W3C */
    position: relative;
    width: 150px;
    height: 100px;
    margin: 10px;
    border: 1px solid #d0d0d0;
    border-radius: 10px;
}

.corner {
    top: -1px;
    right: -1px;
    width: 20px;
    height: 20px;
    background: -moz-linear-gradient(top, #ffffff 0%, #e8e8e8 100%); /* FF3.6+ */
    background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,#ffffff), color-stop(100%,#e8e8e8)); /* Chrome,Safari4+ */
    background: -webkit-linear-gradient(top, #ffffff 0%,#e8e8e8 100%); /* Chrome10+,Safari5.1+ */
    background: -o-linear-gradient(top, #ffffff 0%,#e8e8e8 100%); /* Opera 11.10+ */
    background: -ms-linear-gradient(top, #ffffff 0%,#e8e8e8 100%); /* IE10+ */
    background: linear-gradient(to bottom, #ffffff 0%,#e8e8e8 100%); /* W3C */
    position: absolute;
    border-radius: 0 0 0 5px;
    border-left: 1px solid #d0d0d0;
    border-bottom: 1px solid #d0d0d0;
}

.corner span {
    float: right;
    width: 0;
    height: 0;
    border-top: 20px solid #fff; 
    border-left: 20px solid transparent;
}