JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
    <div class="triangle"></div>
</div>

CSS

.container {
    float: left;
    width: 400px;
    height: 200px;
    margin-top: 25px;
    background-color: blue;
    position: relative;
}
.container .triangle {
     position: absolute;
     top: -25px;
     left: 0;
     border-style: solid;
     border-width: 0 0 25px 400px;
     border-bottom: 25px solid blue;
     border-color: transparent transparent blue transparent;  
}