Идея с использованием трансформации и перспективы, где у вас будет граница, радиус границы и градиент:
body {
background: lightblue;
}
.block {
overflow: hidden;
width: 300px;
height: 200px;
margin: 20px;
position: relative;
z-index:0;
}
.block::before,
.block::after {
content: "";
position: absolute;
z-index:-1;
border: 1px solid #fff;
top: 0;
bottom: 0;
width: 50%;
background-image: linear-gradient(to right, #314b56, #283b44, #1f2c32, #161e21, #0a0f11);
background-size: 200% 100%;
}
.block::before {
left: 0;
border-right: 0;
border-radius: 15px 0 0 15px;
transform-origin: right;
transform: perspective(100px) rotateY(-5deg);
}
.block::after {
right: 0;
border-left: 0;
border-radius: 0 15px 15px 0;
transform-origin: left;
transform: perspective(100px) rotateY(5deg);
background-position: right;
}
<div class="block"></div>
Вы также можете добавить тень и легко изменить градиент:
body {
background: lightblue;
}
.block {
overflow: hidden;
width: 300px;
height: 200px;
margin: 20px;
position: relative;
z-index:0;
filter:drop-shadow(0 0 5px #000);
}
.block::before,
.block::after {
content: "";
position: absolute;
z-index:-1;
border: 1px solid #fff;
top: 0;
bottom: 0;
width: 50%;
background-image: linear-gradient(35deg, blue, red);
background-size: 200% 100%;
}
.block::before {
left: 0;
border-right: 0;
border-radius: 15px 0 0 15px;
transform-origin: right;
transform: perspective(100px) rotateY(-5deg);
}
.block::after {
right: 0;
border-left: 0;
border-radius: 0 15px 15px 0;
transform-origin: left;
transform: perspective(100px) rotateY(5deg);
background-position: right;
}
<div class="block"></div>