* {
  box-sizing: border-box;
}

html, body {
  margin: 0;
  padding: 0;
  min-height: 100%;       /* ensures it fills the viewport */
}

body {
  display: flex;          /* lets you control header/main/footer stacking */
  flex-direction: column; /* vertical layout */
}

main {
  flex: 1;                 /* pushes footer to the bottom */
}