My apologies, I was just basing this on the question relating to SEO specifically and didn’t notice that you prefaced your reply with a focus on accessibility.
That said, I took a peek at the WCAG and I’m not seeing any mention of a requirement to use a single H1 in and of the three Success Criterion sections that speak to heading usage (1.3.1, 2.4.6, and 2.4.10):
The only mention specifically tied to WCAG I could find regarding multiple H1’s is from a teleconference back in 2012 where it was confirmed that it does not violate any of the success criteria:
awk: So long as the heading structures provided match the content, there is no prohibition against an HTML page having multiple H1 heading elements or not following a strict heirarchy of headings, nor any similar prohibition for similar semantic structures in technologies other than HTML.
Loretta: WCAG does not require strict hierarchy in headings. It recommends it but does not require it. Using two
<h1> headings would not violate any of the success criteria, so WCAG conformance would not prohibit it. Success Criterion 1.3.1 requires that if headings are used, they be marked up as headings. It does not require that headings be used at all, or that they be used in any particular way.
A more recent discussion on the topic of multiple H1’s from the Paciello Group reinforces the same logic that, while it may be considered unusual, it doesn’t represent a WCAG failure:
More than one
The converse is also true: a top-heavy heading hierarchy is unusual but not incorrect. Users typically expect only a single
<h1> , which is used to represent the main heading or title on the page. As long as the visual presentation of multiple
<h1> s doesn’t imply a hierarchy (such as the first
<h1> being styled visually to be larger or bolder than the second
<h1> ), then having more than one
<h1> —while potentially confusing— does not represent a WCAG failure.
The main issue I see discussed in the links above is logical hierarchy, however if the content of the page supports it—even if it may not be common practice—you can use more than a single H1 on a page and be WCAG compliant.
Disclaimer: I’m by no means an expert when it comes to accessibility (all of my research regarding the topic was done this morning) so feel free to point me in the direction of something that states otherwise.