See the Pen Untitled by MkEk (@MkEk) on CodePen.
<div class="example"></div>.example {
background-color: #8cc7f6;
box-shadow:
-8px -8px 16px 0px #b9dcf9,
8px 8px 16px 0px #5eb0f2;
}影を内側に向ける場合にはinsetを使用する
.example-inset {
background-color: #8cc7f6;
box-shadow:
inset -8px -8px 16px 0px #b9dcf9,
inset 8px 8px 16px 0px #5eb0f2;
}