SOURCE

<div class="bg"></div>

.bg{
    width: 100px;
    height: 300px;
    background: red;
    position: relative;
    margin: 130px 0 0 130px;
}
.bg::before{
    content: '';
    position: absolute;
    width: 100%;
    height: 100%;
    background: red;
    animation: ss 2s linear infinite;
}
@keyframes ss{
    0%{
        filter:blur(10px);
    }
    25%{
        filter:blur(20px);
    }
    50%{
        filter:blur(40px);
    }
    75%{
        filter:blur(20px);
    }
    100%{
        filter:blur(10px);
    }
}