* {
    box-sizing: border-box;
    /*means adding padding won't change box size*/
}

body {
    font-family: 'Arial';
    font-size: 10pt;
}

div {
    border: 1px solid black;
}

#header {
    width: 1000px;
    height: 100px;
    padding: 20px;
    margin-left: auto;
    margin-right: auto;

}

#container {
    display: flex;
    width: 1000px;
    margin-left: auto;
    margin-right: auto;
}

#sidebar {
    padding: 20px;
}

#sidebar a {
    display: block;
}

#footer {
    width: 1000px;
    margin-left: auto;
    margin-right: auto;
}

#header,
#footer,
#sidebar,
#content {
    padding: 20px;
}