Семантические теги header, main, footer

Допускается, их может быть несколько на странице или вообще не быть. Вырезки из спеки:

<header>

http://w3c.github.io/html/sections.html#the-header-element

A header element is intended to usually contain the section’s heading (an h1–h6 element), but this is not required. The header element can also be used to wrap a section’s table of contents, a search form, or any relevant logos.

<footer>

http://w3c.github.io/html/sections.html#the-footer-element

The footer element represents a footer for its nearest ancestor main element or sectioning content or sectioning root element. A footer typically contains information about its section, such as who wrote it, links to related documents, copyright data, and the like.

<main>

Более того, начиная с HTML 5.2 может быть несколько main

http://w3c.github.io/html/grouping-content.html#the-main-element

A document must not have more than one main element that does not have the §5.1 The hidden attribute attribute specified.