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I think this can't be better explained:

css font sizing by @ireaderinokun

Summing up:

html {
  font-size: 62.5%; /* sets the base font to 10px for easier math */
}

body {
  font-size: 16px;
  font-size: 1.6rem;  
  /* sets the default sizing to make sure nothing is actually 10px */
}

h1 {
  font-size: 32px;
  font-size: 3.2rem;
}

Then use media queries:

@media screen and (min-width: 1280px) {
  html {
    font-size: 100%;
  }
}