body {
    max-width: 800px;
    margin: auto;
}

#parts {
    float: left;
    width: 45%;
    /*display: inline-block; /*inline-block*/
    border: 2px solid black;
    margin-left: 1%;
}

#assembl {
    float: right;
    width: 45%;
    /*height: 800px;*/
    /*margin-bottom: 100px;*/
    /*margin-left: 1%;*/
    /*position: fixed;
    bottom: 10%;
    right: 5%;*/
    top: 10%;
    border: 2px solid black;
}

#assembl img {
    border-radius: 10px;
    width: 90%;
    margin-left: auto;
    margin-right: auto;
    margin-top: 10%;
}

h2 {
    color:red;
    text-align: center;
}

#parts img {
    border-radius: 10px;
    width: 100%;
    margin-left: auto;
    margin-right: auto;
}