JSFiddle - React, Tailwind, and code Playground

HTML

<button>button</button>

CSS

button {
    border: 0 none;
    background-color: red;
    position: relative;
    
    font-size: 4em;
    margin-left: 100px; 
}

button:before,
button:after {
    content: '';
    position: absolute;
    background-color: inherit;
    top: 0;
    bottom: 0;
    width: 1em;
    z-index: -1;
}

button:before {
    left: -0.5em;
    -webkit-transform: skew(-10deg);
    transform: skew(-10deg);
    border-top-left-radius: 10%;
}

button:after {
    left: auto;
    right: -0.5em;
    -webkit-transform: skew(10deg);
    transform: skew(10deg);
    border-top-right-radius: 10%;
}