Use this code:

Good Style:
<H1>Level One First</H1>
<H2>Level Two A</H2>
<H2>Level Two B</H2>
<H3>Level Three AA</H3>
<H3>Level Three BB</H3>
<H1>Level One Second</H1>
Bad Style:
<H1>Level One First</H1>
<H4>Level Four A</H4>
<H3>Level Three B</H3>
<H6>Level Six AA</H6>
<H3>Level Three BB</H3>
<H1>Level One Second</H1>

And get this result:

Good Style:

Level One First

Level Two A

Level Two B

Level Three AA

Level Three BB

Level One Second

Bad Style:

Level One First

Level Four A

Level Three B

Level Six AA

Level Three BB

Level One Second