html{
    height: 100%;
}
body {
    min-height: 100%;
    display: grid;
    grid-template-columns: auto;
    grid-template-rows: auto 1fr auto;
    grid-template-areas: 
    "header"
    "content"
    "footer";
}
header {
    grid-area: header;
}
main {
    grid-area: content;
    position: relative;
}
footer {
    grid-area: footer;
}