By the way I notice the page [developer.mozilla.org ] suggests putting the h1 tag in the header element of the body.
Header is a Semantic Element (see [
w3schools.com ]) in HTML5.
H1 - H6, which are hierarchical, can be used with or without HTML5 semantic markup, but also have a semantic function (see [
w3.org ], as well as the W3C schools link in keyplyr's post), and are in my view much more important for both users and SEs than the new Semantic Elements: H1 is necessary, but Header isn't. H1 can go inside a Header element, but it doesn't have to.
H1 used to be important for SEO, and while it is probably a lot less significant now, headings still can (and if used corrrectly should) help search engines capture the subject and meaning of a page. They address what a page and its sections are
about. While "keywords" may not be what they once were, it is still good practice to make it as clear as possible that a page about dog-grooming (H1) is about dog-grooming, that the first part (H2) is about dog-grooming techniques, the second (H2) about dog-grooming equipment, and the third (H2) about whatever else. If any of those parts have subsections, then use H3, etc. Although H6 is probably going further than you need, at least H1- H3 or H4 are pretty useful.
Header, Article, Section, etc. don't tell SEs what a section is
about, but what its
function is: <article> means "this is an article", <header> means "this section contains summary and navigatiion information", while <h1> means "this is the page topic".
Keep these distinctions in mind and you won't go far wrong.