JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
	<div id="masque">
		<span class="triangle triangleLeft"></span>
		<span class="triangle triangleTop"></span>
	</div>
</div>

CSS

*{
	margin: 0;
	padding: 0;
	border: 0;
}

#container{
	position: relative;
	min-height: 250px;
	height: 50vh;
	background: url("http://i.imgur.com/4FQ6LwN.jpg?1") no-repeat fixed center;
}

	#masque{
		position: absolute;
		bottom: 0;
		left: 10vw;
		right: 0;
		height: 10vw;
		background: white;
	}

		.triangle{
			position: absolute;
			width: 0;
			height: 0;
			border-bottom: 10vw solid white;
			border-left: 10vw solid transparent;
		}
			.triangleLeft{bottom: 0; left: calc(-10vw + 1px);}
			.triangleTop{bottom: 10vw; right: 0;}