JSFiddle - React, Tailwind, and code Playground

HTML

<div class="block">Кнопка</div>

CSS

.block{
    position: relative;
    display: inline-block;
    margin: 50px;
    color: #222;    
    background: #777;
    text-shadow: 0 1px 1px #888;
    font-family: Arial;
    font-size: 16px;
    font-weight: bold;
    width: 50%;
    max-width: 500px;
    text-align: center;
}

.block:before, .block:after{
    content:'';
    position: absolute;
    left: 0;
    top: -40px;
    z-index: -1;
    background: #777;
    padding: 50% 0;
    width: 100%;
    border-radius: 8% 8% 0 0;
    clip: rect(0, auto, 40px, 0);
}

.block:after {
    -webkit-transform: scaleY(-1);
    -moz-transform: scaleY(-1);
    -ms-transform: scaleY(-1);
    -o-transform: scaleY(-1);
    transform: scaleY(-1);
    top: auto;
    bottom: -40px;
}