Square Illusion

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<div class="container">
    <div class="square  square-1"></div>
    <div class="square  square-2"></div>
    <div class="circle  circle-1"></div>
    <div class="circle  circle-2"></div>
    <div class="circle  circle-3"></div>
    <div class="circle  circle-4"></div>
</div>

html {
    box-sizing: border-box;
}
*,
*:before,
*:after {
    box-sizing: inherit;
}
html,
body {
    height: 100%;
}
body {
    background-color: black;
    position: relative;
    overflow: hidden;
}
.container {
    width: 30%;
    width: 25vw;
    height: 30%;
    height: 25vw;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate3d(-50%, -50%, 0);
    cursor: help;
}
.square {
    width: 70%;
    height: 100%;
    position: absolute;
    top: 0;
    left: 50%;
    transform: translateX(-50%);
    border-top: 10px solid mediumseagreen;
    border-top: 1.5vw solid mediumseagreen;
    border-bottom: 10px solid mediumseagreen;
    border-bottom: 1.5vw solid mediumseagreen;
    animation-duration: 1s;
    animation-iteration-count: infinite;
    animation-timing-function: linear;
}
.square-1 {
    transform: translateX(-50%) rotate(45deg);
    animation-name: square-1;
}
.square-2 {
    transform: translateX(-50%) rotate(-45deg);
    animation-name: square-2;

}
@keyframes square-1 {
    0%, 100% {
        transform: translateX(-60%) translateY(7%) rotate(45deg);
    }
    25%, 75% {
        transform: translateX(-50%) translateY(0) rotate(45deg);
    }
    50% {
        transform: translateX(-40%) translateY(-7%) rotate(45deg);
    }
}

@keyframes square-2 {
    0%, 50%, 100% {
        transform: translateX(-50%) translateY(0) rotate(-45deg);
    }
    25% {
        transform: translateX(-40%) translateY(7%) rotate(-45deg);
    }
    75% {
        transform: translateX(-60%) translateY(-7%) rotate(-45deg);
    }
}

.circle {
    background-color: coral;
    opacity: 0;
    width: 30%;
    width: 9vw;
    height: 30%;
    height: 9vw;
    text-align: center;
    font-size: 5vw;
    line-height: 9vw;
    position: absolute;
    overflow: hidden;
    text-indent: 100%;
    border-radius: 50%;
    transition: opacity .2s;
    animation-duration: 4s;
    animation-iteration-count: infinite;
    animation-timing-function: linear;
    .container:hover &,
    .container:focus & {
        opacity: 1;
    }
    &:hover,
    &:focus {
        text-indent: 0;
    }
}
.circle-1 {
    top: 0;
    left: 50%;
    transform: translateX(-50%) translateY(-100%);
}
.circle-2 {
    bottom: 0;
    left: 50%;
    transform: translateX(-50%) translateY(100%);
}
.circle-3 {
    top: 50%;
    left: 0;
    transform: translateX(-100%) translateY(-50%);
}
.circle-4 {
    top: 50%;
    right: 0;
    transform: translateX(100%) translateY(-50%);
}

.container:before,
.container:after {
    color: white;
    font-size: 3vw;
    white-space: nowrap;
    position: absolute;
    top:  50%;
    left: 50%;
    transform: translate3d(-50%, -50%, 0);
    transition: opacity .5s;
}
.container:before {
    content: "Hover me!";
    opacity: .8;
}
.container:hover:before {
    opacity: 0;
}
.container:after {
    content: "Whoa...";
    opacity: 0;
}
.container:hover:after {
    opacity: .5;
}