flex-reset.css

* { box-sizing: border-box; }
html, body { min-height: 100vh; margin: 0; }

div,
html,
body,
nav,
header,
hgroup,
footer,
section,
article,
ul,
li,
a,
span,
button {
  position: relative;
  display: flex;
  flex-direction: column;
  flex-shrink: 0;
}

a,
span,
button,
nav {
  flex-direction: row;
}

a,
span,
button {
  align-items: center;
  display: inline-flex;
}

.center {
  align-items: center;
  justify-content: center;
}