Concave corners

:root {
                        --concave-corners-background: rgba(0, 0, 0, 0.3);
                        --concave-corners-size: 1rem;
                    }

                    .concave-corners {
                        background-color: var(--concave-corners-background);

                        /* Used to position the corners */
                        position: relative;

                        /* Misc */
                        height: 100%;
                    }

                    .concave-corners__corner {
                        /* Absolute position */
                        position: absolute;

                        /* Size */
                        height: var(--concave-corners-size);
                        width: var(--concave-corners-size);

                        background: #fff;
                    }

                    .concave-corners__corner--tl {
                        /* Position */
                        left: 0;
                        top: 0;

                        /* Border radius */
                        border-radius: 0 0 var(--concave-corners-size) 0;
                    }

                    .concave-corners__corner--tr {
                        /* Position */
                        right: 0;
                        top: 0;

                        /* Border radius */
                        border-radius: 0 0 0 var(--concave-corners-size);
                    }

                    .concave-corners__corner--bl {
                        /* Position */
                        bottom: 0;
                        left: 0;

                        /* Border radius */
                        border-radius: 0 var(--concave-corners-size) 0 0;
                    }

                    .concave-corners__corner--br {
                        /* Position */
                        bottom: 0;
                        right: 0;

                        /* Border radius */
                        border-radius: var(--concave-corners-size) 0 0 0;
                    }