JSFiddle - React, Tailwind, and code Playground

HTML

<div><button>Новый расчет</button></div>

CSS

* {
    padding: 0;
    margin: 0;
}
body {
    background: #f0f4f6;
}
div {
    width: 13em;
    height: 2.7em;
    background: #313131;
    z-index: -1000;
    position: absolute;
}
button {
    text-transform: uppercase;
    padding: 0.5em;
    width: 11em;
    border: 1px solid #639828;
    border-left: 0;
    background: linear-gradient(to bottom, #a9c04d 0%, #639828 100%);
    color: white;
    text-shadow: 0 -1px 0 black;
    box-shadow: inset 0 1px 0 #d5e0a6, 5px 5px 10px -5px rgba(0, 0, 0, 0.5);
    margin-left: 2em;
    margin-top: 1em;
    font-size: 1em;
    position: absolute;
}
button:before {
    content: "";
    position: absolute;
    top: 4px;
    left: calc(-1em + 3px);
    height: calc(2em - 8px);
    width: calc(2em - 8px);
    -webkit-transform: rotate(45deg);
    background: linear-gradient(1225deg, #a9c04d 0%, #639828 100%);
    border: 1px solid #639828;
    box-shadow: inset 1px 0 0 #d5e0a7, 5px -3px 10px rgba(0, 0, 0, 0.5);
    z-index: -100;
}