I used to be no CSS, but a little CSS can improve readability. There's nothing magical about the browser default fonts, margins, etc. And no max-width can be problematic.

You might want this to prevent the text size from changing when you rotate to landscape on mobile:

  html {
    -webkit-text-size-adjust: none;
    text-size-adjust: none;
    -moz-text-size-adjust: none;
  }