css3凹角效果

<!DOCTYPE html>
<html lang="en">
<head>
    <meta http-equiv="Content-Type" content="text/html;charset=UTF-8">
    <title>css</title>
</head>
<body>
<style>
#a {
    width: 150px;
    height: 300px;
    background: radial-gradient(circle at top left,
    transparent 15px, #e6e4b7 0) top left,
    radial-gradient(circle at top right,
            transparent 15px, #e6e4b7 0) top right,
    radial-gradient(circle at bottom right,
            transparent 15px, #e6e4b7 0) bottom right,
    radial-gradient(circle at bottom left,
            transparent 15px, #e6e4b7 0) bottom left;
    background-size: 50% 50%;
    background-repeat: no-repeat;
    padding: 20px 24px 22px 20px;
    filter: drop-shadow(2px 2px 10px red);
}
#b {
    display: block;
    background: radial-gradient(circle at top left,
    transparent 15px, red 0) top left,
    radial-gradient(circle at top right,
            transparent 15px, red 0) top right,
    radial-gradient(circle at bottom right,
            transparent 15px, red 0) bottom right,
    radial-gradient(circle at bottom left,
            transparent 15px, red 0) bottom left;
    background-size: 50% 50%;
    background-repeat: no-repeat;
    width:100%;
    height: 100%;
    padding: 2px;
}
#b:before {
    content: "";
    width: 100%;
    height: 100%;
    display: block;
    background: radial-gradient(circle at top left,
    transparent 15px, #e6e4b7 0) top left,
    radial-gradient(circle at top right,
            transparent 15px, #e6e4b7 0) top right,
    radial-gradient(circle at bottom right,
            transparent 15px, #e6e4b7 0) bottom right,
    radial-gradient(circle at bottom left,
            transparent 15px, #e6e4b7 0) bottom left;
    background-size: 50% 50%;
    background-repeat: no-repeat;
}
</style>

    <div id="a">
        <div id="b"></div>
    </div>
</body>
</html>

内凹圆角的实现

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background: radial-gradient(circle at top left, transparent 15px, #58a 0) top left,             radial-gradient(circle at top right, transparent 15px, #58a 0) top right,             radial-gradient(circle at bottom left, transparent 15px, #58a 0) bottom left,             radial-gradient(circle at bottom right, transparent 15px, #58a 0) bottom right; background-repeat: no-repeat; background-size: 50% 50%;