Example

Heading 1

Heading 2

Heading 3

Heading 4

Heading 5
Heading 6
Markup
  <h1>Heading 1</h1>
  <h2>Heading 2</h2>
  <h3>Heading 3</h3>
  <h4>Heading 4</h4>
  <h5>Heading 5</h5>
  <h6>Heading 6</h6>
Source: text/fonts.less, line 122