There are three <h1> tags on pages and single posts when using the default templates. It is recommended to start the main content with <h1> for better accessibility. Also, the accessibility requirements require themes to have no more than one <h1> tag on a single page.
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Consider using <p> for the site title in the footer instead of <h1>
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For the site title in the header, the classic themes traditionally have the mechanism of having <h1> on the homepage, <div> on all other pages. FSE themes could have multiple headers to achieve this, but it’s not ideal. Perhaps this is a concern with GB.