body {
    background-image: url(background.png);
    background-position: top left;
    background-repeat: no-repeat;
    background-size: contain;
}

main {
    position: absolute;
    display: grid;
    top: 0;
    right: 0;
    bottom: 0;
    overflow-y: auto;
    padding: 0 10px;
    background-color: white;
}

header {
    color: #0039a6;
    text-align: center;
}

header p {
    text-align: center;
}

#welcome {
    text-align: justify;
    padding: 0 10px;
}

#welcome h3 {
    color: #d52b1e;
    text-align: center;
    margin-bottom: 10px;
}

#welcome p {
    text-indent: 20px;
}

nav {
    border-top: 2px solid #d52b1e;
}

nav a {
    padding: 0 10px;
    display: block;
}

nav a:hover {
    background-color: #d52b1e;
    color: white;
}

@media (max-width: 799px) {
    
   main {
        top: 50vw;
        align-content: start;
    }
    
    main welcome {
        overflow-y: auto;
    }    
}

@media (max-width: 799px) and (max-height: 599px) {    
    body {
        background: none;
    }
    
    main {
        top: 0;
        left: 0;
    }    
}

@media (max-height: 599px) and (min-width: 800px){
    
    body {
        background-size: auto 100%;
    }
    
    main {
        left: 100vh;
        max-width: 900px;
    }    
}

@media (max-height: 599px) and (min-aspect-ratio: 4/1) {
    
    main {
        left: 200vh;
        max-width: 900px;
    }  
    
}

@media (max-height: 199px) and (max-aspect-ratio: 2/1) {
    
    body {
        background: none;
    }
    
    main {
        left: 0;
    }
    
}

@media (max-width: 799px) and (min-width: 400px) and (min-height: 600px) {    
    
    body {
        padding-top: 50vw;
    }
    
    main {
        display: block;
        top: 50vw;
    }
    
    #welcome {
        padding-bottom: 20px;
    }
    
}

@media (min-height: 600px) and (min-width: 800px) and (max-aspect-ratio: 4/3) {
    
    main {
        top: 50vw;
        grid-template-columns: 200px auto;
        grid-template-rows: 70px auto;
        grid-template-areas: "nav header"
                             "nav welcome";
    }
    
    main header {
        grid-area: header;
    }
    
    main nav {
        padding-top: 10px;
        grid-area: nav;
        border-top: none;
        border-right: 2px solid red;
    }
    
    #welcome {
        grid-area: welcome;
    }
    
}

@media (max-width: 399px) and (min-height: 599px) { /* Вертикальный столбик */
    
    
    body {
        background: none;
        padding: 0;
    }
    
    main {
        top: 0;
        justify-content: center
    }

}

@media (min-height: 600px) and (min-width: 800px) and (min-aspect-ratio: 4/3) and (max-aspect-ratio: 2/1)  {
    
    body {
        background-size: cover;
    }
    
    main {
        top: 0;
        left: 0;
        background-color: transparent;
    }

    header {
        padding-top: 3vh;
        position: absolute;
        right: 6vw;
        font-size: 3vh;
    }
    
    #welcome {
        position: absolute;
        top: 34vh;
        left: 50vw;
        right: 5vw;
        font-size: 1.5vh;
        grid-area: none;
    }
    
    #welcome h3 {
        font-size: 1.5em;
    }

    main nav {
        position: absolute;
        grid-area: none;
        top: 63vh;
        left: 95vh;
        bottom: 8vh;
        font-size: 3vh;
        display: flex;
        flex-direction: column;
        justify-content: space-between;
        border: none;
    }    
}

@media (min-aspect-ratio: 4/3) and (min-height: 600px) {    
    header {
        padding-top: 2vh;
        font-size: 3vh;
    }
}

@media (min-height: 600px) and (min-width: 800px) and (min-aspect-ratio: 2/1) {

    body {
        padding-left: 100vh;
    }
    
    main {
        position: relative;
        height: 100%;
        max-width: 200vh;
        background-color: white;
    }
}