html, body {
    font-size: 1.2rem;    
}

h1, h3 {
    font-size: 5rem;
    margin: auto;
    max-width: 80%;
}

main {
    font-size: 1rem;
}


@media all and (-ms-high-contrast: none), (-ms-high-contrast: active) {
   /* IE10+ CSS */
   h1, h3 {
       margin: inherit;
   }
}