JSFiddle - React, Tailwind, and code Playground

HTML

<div class="callout">
    This is a callout box!
    <b class="notch"></b>
</div>

CSS

.callout {
    position: relative;
    margin: 18px 0;
    padding: 18px 20px;
    background-color: #eef4f9;
    /* easy rounded corners for modern browsers */
    -moz-border-radius: 6px;
    -webkit-border-radius: 6px;
    border-radius: 6px;
}
.callout .notch {
    position: absolute;
    top: -10px;
    left: 20px;
    margin: 0;
    border-top: 0;
    border-left: 10px solid transparent;
    border-right: 10px solid transparent;
    border-bottom: 10px solid #eef4f9;
    padding: 0;
    width: 0;
    height: 0;
    /* ie6 height fix */
    font-size: 0;
    line-height: 0;
     /* ie6 transparent fix */
    _border-right-color: pink;
    _border-left-color: pink;
    _filter: chroma(color=pink);
}