CSS3 四边形凹角写法

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .outer{
            position: absolute;
            left: 100px;
            top: 100px;
            width: 300px;
            height: 500px;
            background-color: #0a6ebd;
            display: flex;
            justify-content: center;
            align-items: center;
            box-sizing: border-box;
            border: 1px solid #0a6ebd;
        }
        .inner-1{
            width: 250px;
            height: 450px;
            box-sizing: border-box;
            background-color: #0da6ec;
            position: relative;
            display: flex;
            justify-content: center;
            align-items: center;
            border: 1px solid red;
            border-top-left-radius: 25px;

        }
        .inner-2{
            width: 200px;
            height: 400px;
            box-sizing: border-box;
            background-color: #00bcd4;
            position: relative;
            border: 1px solid red;
            border-top-left-radius: 25px;

        }
        .left-corner{
            width: 25px;
            height: 25px;
            /* border-radius: 100%; */
            border-bottom-right-radius: 100%;
            background-color: inherit;
            position: absolute;
            left: -1px;
            top: -1px;
            z-index: 10;
            border-bottom: 1px solid red;
            border-right: 1px solid red;
            box-sizing: border-box;
        }
    </style>
</head>
<body>

    <div class="outer">

        <div class="inner-1">
            <div class="left-corner" style="background-color: #0a6ebd;"></div>
            <div class="inner-2">
                <div class="left-corner" style="background-color: #0da6ec;"></div>
            </div>
        </div>
    </div>
</body>
</html>
CSS3 四边形 凹角写法

CSS3 四边形凹角写法
