Try using the new-blocklevel-tags parameter, which specifies the new HTML5 tags. You can include multiple elements by separating them with a comma or space. You should get the expected result without errors, but there will still be warnings.
Try this in cq:
xdmp:tidy(xdmp:document-get("home.html"), <options xmlns="xdmp:tidy"><new-blocklevel-tags>header section</new-blocklevel-tags></options>)
( , , ), xdmp: tidy. , , !