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.block {
width: 300px;
margin: 70px auto 0;
}
.a {
position: relative;
width: 100%;
height: 300px;
}
.a span {
position: absolute;
}
.a span:nth-child(1),.a span:nth-child(4) {
top: 0;
background-color: #7f7f7f;
transition: all .9s linear;
}
.a span:nth-child(3),.a span:nth-child(2) {
left: 0;
background-color: #7f7f7f;
transition: all .9s linear;
}
.a span:nth-child(1) {
left: 0;
width: 0;
height: 1px;
transform-origin: left center;
}
.a span:nth-child(2) {
top: 0;
width: 1px;
height: 100%;
transform-origin: center top;
transform: scaleY(0);
}
.a span:nth-child(3) {
bottom: 0;
width: 0;
height: 1px;
transform-origin: left center;
transition-delay: .9s;
}
.a span:nth-child(4) {
right: 0;
width: 1px;
height: 100%;
transform: scaleY(0);
transform-origin: center top;
transition-delay: .9s;
}
.block.active .a span:nth-child(3),
.block.active .a span:nth-child(1) {
width: 100%;
}
.block.active .a span:nth-child(4),
.block.active .a span:nth-child(2) {
transform: scaleY(1);
}
@media screen and (max-width:640px){
.block {
width: 85%;
}
}
$(window).on('load', function(){
$(".block").addClass("active");
});