JSFiddle - React, Tailwind, and code Playground

HTML

<body>
    <div class="menu">
        <div class="main"></div>
        <div class="cut"></div>
    </div>
</body>

CSS

body { font-size: 0; background: yellow; }

.menu { background: transparent; }

.main
{
    display: inline-block;
    width: 200px;
    height: 100px;
    background: red;
}

.cut
{
    display: inline-block;
    border: 50px solid transparent;
    border-top: 50px solid red;
    border-left: 50px solid red;
}