header {
    height: 52px;
    background-color: #baffff91;
}

body {
    margin: 0px;
    position: relative;
}
aside {width: 300px;height: 100vh;position: absolute;transition: left 0.25s ease;top: 0px;}

aside + main {
    display: block;
    float: left;
    width: 100%;
    transition: margin 0.25s ease;top: 0px;
}

aside:not(.active) {
    left: -300px;
}

aside.active + main {
    margin-left: 300px;
}