使用box-shadow/渐变实现内切角，HTML/Vue代码在线实时编辑、预览

            
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            body {
    background: linear-gradient(90deg, #fff, #bbb);
}

div {
    position: relative;
    width: 20vw;
    height: 8vw;
    margin: 1vw auto;
    border-radius: 1vmin;
    // background: #e91e63;
    overflow: hidden;
    line-height: 8vw;
    color: #fff;
    text-align: center;
}

.shadow {
    
    &::before {
        position: absolute;
        content: "";
        top: -2vw;
        left: -2vw;
        width: 4vw;
        height: 4vw;
        border-radius: 50%;
        box-shadow: 0 0 0 25vw #e91e63; 
        z-index: -1;
        animation: shadowmove 10s infinite;
    }
}

.shadow2 {
        &::before {
        position: absolute;
        content: "";
        top: -2vw;
        left: -2vw;
        width: 4vw;
        height: 4vw;
        border-radius: 50%;
        box-shadow: 0 0 0 15vw #e91e63; 
        z-index: -1;
    }
    
    &::after {
        position: absolute;
        content: "";
        bottom: -2vw;
        right: -2vw;
        width: 4vw;
        height: 4vw;
        border-radius: 50%;
        box-shadow: 0 0 0 15vw #e91e63; 
        z-index: -1;
    }
}

@keyframes shadowmove {
    0%{
        background: #e91e63; 
        box-shadow: 0 0 0 0 #e91e63; 
    }
    
    10% {
        background: transparent; 
        box-shadow: 0 0 0 0 #e91e63; 
    }
    
    50% {
        background: transparent; 
        box-shadow: 0 0 0 25vw #e91e63; 
    }
}

.linear {
    background-size: 100% 100%;
    background-image: radial-gradient(circle at 0 0, transparent 0, transparent 2vw, #03A9F5 2vw);
    background-repeat: no-repeat;
}

.linear2 {
    filter: drop-shadow(0 0 3px #666);
    background-size: 70% 70%;
    background-image: 
        radial-gradient(circle at 100% 100%, transparent 0, transparent 2vw, #03A9F5 2vw),
        radial-gradient(circle at 0 0, transparent 0, transparent 2vw, #03A9F5 2vw),
        radial-gradient(circle at 100% 0, transparent 0, transparent 2vw, #03A9F5 2vw),
        radial-gradient(circle at 0 100%, transparent 0, transparent 2vw, #03A9F5 2vw);
    background-repeat: no-repeat;
    background-position: right bottom, left top, right top, left bottom;
}