Pomysł mocno kombinowany i mam wrażenie, że trochę "na neandertala", Tło + 2 białe trójkąty w css trójkąty zasłaniające fragmenty tla.
index.html
<!DOCTYPE html>
<html>
<head>
<meta charset='utf-8'>
<meta http-equiv='X-UA-Compatible' content='IE=edge'>
<title>Trójkąt</title>
<meta name='viewport' content='width=device-width, initial-scale=1'>
<link rel='stylesheet' type='text/css' media='screen' href='main.css'>
</head>
<body>
<div class='city'>
</div>
<div class='triangle-left'>
</div>
<div class='triangle-right'></div>
</body>
</html>
main.css
*{
margin: 0;
padding: 0;
}
.city {
background-image: url('city.jpg');
background-repeat: no-repeat;
background-size: cover;
background-position: right top;
height: 500px;
width: 100vw;
}
.triangle-left {
position: relative;
top: -100px;
width: 0;
height: 0;
border-bottom: 100px solid white;
border-right: 50vw solid transparent;
}
.triangle-right {
position: relative;
top: -200px;
left: 50vw;
width: 0;
height: 0;
border-bottom: 100px solid white;
border-left: 50vw solid transparent;
}