* {
    box-sizing: border-box;
    border: 1px solid black;
}
body {
    width: 98%;
    margin: auto;
}
section, article, aside, footer, header {
    padding: 0.5rem;
}
img, video, embed {
    max-width: 98%;
}

header{
    text-align: center;
    flex:33%;
    border-right: solid black 3px;
    border-bottom: solid black 3px;
    background-color: gold;
}
.nav{
    
    display:flex;
    text-align:center;
    flex-direction:column;
    list-style-type:none;
    background-color: black;
}

.nav, header{
    
    order:-1;
    flex-direction:row;
    
}

.nav {
    
    display:inline; 
    flex:auto;
    text-align:center;
    background: #ccc;
    color: black;
    margin: 0 5px;
    padding: 5px 0;
    text-decoration:none;
    border: 1px solid black;
    
}

.nav:hover{
    outline: 1px solid blue;
    color: blue;
    text-decoration: underline;
    flex-grow:2;
    
}
.parent{
    
    display:flex;
    flex-wrap: wrap;
    border-bottom:0.5rem;
    border-left: solid black 3px;
    border-right: solid black 3px;
    text-align:center;
    
}

.a1{
    
    flex:33%;
    border-right: solid black 3px;
    border-bottom: solid black 3px;
    background-color: gold;
    
}

.a2{
    
    flex:33%;
    border-right: solid black 3px;
    border-bottom: solid black 3px;
    background-color: gold;
    
}

.aside{
    flex:33%;
    border-right: solid black 3px;
    border-bottom: solid black 3px;
    background-color: gold;
}
.section{
    flex:33%;
    border-right: solid black 3px;
    border-bottom: solid black 3px;
    background-color: gold;
}
.footer{
    flex:33%;
    border-right: solid black 3px;
    border-bottom: solid black 3px;
    background-color: gold;
}

/* default navigation values */
nav {
    display: flex;
    flex-direction: column;
}
nav a {
    flex: 100%;
    margin: 0 5px;
    padding: .5rem;
}

/* medium screen */
@media screen and (min-width: 768px) {
    body {
        display: flex;
        flex-direction: column;
        max-width: 75rem;
    }
    main {
        display: flex;
        flex-wrap: wrap;
    }
    nav, header {
        order: -1;
    }
    nav {
        flex-direction: row;
    }
    nav a {
        flex: auto;
    }
    article {
        flex: 50%;
    }
    aside, section {
        flex: 100%;
    }
    section p {
        column-count: 2;
        column-gap: 20px;
        column-rule: 1px solid rgb(255, 105, 0);
    }
}

/* large screen */
@media screen and (min-width: 1200px) {
    article {
        flex: 35%;
    }
    aside {
        flex: 30%;
    }
}