Edit in JSFiddle

h1 {
  font-size: 30px;
}

h2 {
  font-size: 1.875em;
}

p {
  font-size: 0.875em;
}
<!DOCTYPE html>
<html lang="ko">
  <head></head>
  <body>
    <h1>This is heading 1</h1>
    <h2>This is heading 2</h2>
    <p>em 단위는 W3C 권장 단위입니다.</p>
  </body>
</html>