css.css

/* body {
    margin: 0;
    height: 100%;
} */

.main-part {
    width: 806px;
    height: 563px;
    position: relative;
}

.first-div {
    background-color: #daa1df;
    position: static;
    color: black;
    width: 100px;
    height: 60px;
    top: 10px;
    left: 10px;
}

.second-div {
    position: relative;
    background-color: #f1a358;
    color: black;
    left: 100px;
    top: 20px;
    width: 200px;
    height: 200px;
}

.third-div {
    position: fixed;
    background-color: #99ed8a;
    width: 200px;
    height: 100px;
    top: 20px;
    left: 300px;
    z-index: 100;
    border: 1px solid green
}

.parent-block {
    position: relative;
    background-color: #c0c0c0;
    border: 1px solid black;
    width: 250px;
    height: 250px;
    left: 300px;
    top: 20px;
}

.child-block {
    border: 1px solid red;
    background-color: #ec807e;
    width: 80px;
    height: 80px;
    top: 50px;
    left: 80px;
    position: absolute;
}