main.css

@import url('https://fonts.googleapis.com/css?family=Poppins');
html {
    font-family: 'Poppins', sans-serif;
    letter-spacing: 1px;
}

.row {
    display: flex;
        position: absolute;
        top: 50%;
        left: 50%;
        transform: translate(-50%, -50%);
}

.column {
    flex: 50%;
}

body {
    background-color: #81c784;
}

div {
    margin: auto;
    width: 50%;
    line-height: 50px;
}

img {
    position: relative;
    display: block;
    width: 90%;
    margin-left: auto;
    margin-right: auto;
}

.demo {
    background-image: url("images/demo.gif");
    background-position: center;
    background-repeat: no-repeat;
    background-size: 95%;
}

h1 {
    position: relative;
    text-align: center;
    line-height: 0px;
    font-size: 5vw;
    color: #FFF
}

p {
    position: relative;
    text-align: center;
    line-height: 0px;
    font-size: 3vw;
    color: #FFF;
}

.copyright {
    position: absolute;
    top: 95%;
    left: 50%;
    transform: translate(-50%, -50%);
}

.copyright p {
    font-size: 10px;
}

@media screen and (max-width: 600px) {
    div {
        width: 300px;
    }
    h1 {
        font-size: 30px;
        /* 600 * 5% */
    }
    p {
        font-size: 18px;
        /* 600 * 3% */
    }
}

@media screen and (min-width: 900px) {
    img {
        width: 202.5px;
        /* 900 * 50% * 45%  */
    }
    h1 {
        font-size: 45px;
        /* 900 * 5% */
    }
    p {
        font-size: 27px;
        /* 900 * 3% */
    }
}