3.5 Use header elements to convey document structure.....

Translation

Use header levels (H1-H6) to specify the document structure and use them as specified.

Means

E.g.:

• Each page should start with H1.
• Use H2 to indicate a subsection of H1.
• H3 must not come before H2, the header levels must be used in the correct order.
• However, after H5 you can use any header level.
• Do not use header markers to create font effects.

Make sure that the headers are marked with header levels.