@charset "utf-8";
html,
body{
    width:100%;
    height:100%;
}
header{
    width:100%;
    height:5%;
    background-color: aqua;
}
main{
    width:100%;
    height:200%;
}
main .item{
    width:100%;
    height:25%;
    background-color: green;
}
footer{
    width:100%;
    position:fixed;
    bottom:0;
    height:0;
    background-color:yellow;
    transition:300ms;
    overflow:hidden;
}
footer.fixed{
    position:fixed;
    left:0;
    bottom:0;
    height:20%;
    transition:300ms;
}