Why it matters
Google uses the first visible h1 as a title-link source and Bing expects the h1 to reflect the title; agents and Markdown converters use it as the document title.
How to fix it
Render one <h1> per page on the server, describing the page topic. Use h2–h6 for sections and don't put the logo in an h1.
References
More seo rules
void seo reports 60 rules in this category. What search engines need from the raw response: status codes, titles and descriptions in <head>, self-referencing canonicals, one h1, crawlable links, Open Graph, icons, robots.txt and a sitemap with real dates.
canonical-brokenCanonical points to a URL that returns 4xx/5xxcanonical-conflictCanonical target redirects, is noindex, or canonicalises elsewherecanonical-not-selfCanonical points to a different URLhead-js-mutationJavaScript changes canonical, robots, title or description after loadh1-title-mismatch<h1> and <title> share little wordingheading-skipHeading levels skip (h2 → h4) or headings are emptyimg-alt-missing<img> without an alt attributeimg-alt-qualityAlt text is a filename, a generic word, or overly long