What BIMI is
BIMI (Brand Indicators for Message Identification) is a way to publish a brand logo for e-mail. The domain publishes a TXT record at default._bimi.<domain>, such as v=BIMI1; l=https://example.com/logo.svg; a=https://example.com/vmc.pem. The l tag is the logo, an SVG in the restricted "SVG Tiny PS" profile, and the optional a tag points to a mark certificate (VMC or CMC) that ties the logo to the organization.
A mailbox provider only considers the record for mail that passes DMARC, from a domain whose DMARC policy is enforced: p=quarantine or p=reject, applied to all mail. A domain with p=none does not qualify, whatever its BIMI record says.
What this tool checks
It reads the BIMI record for the selector you give (default when empty), checks that it starts with v=BIMI1 and has an l tag, requests the logo URL and reports whether it is HTTPS, answers with status 200 and the content type image/svg+xml, whether the file declares the tiny-ps base profile, and how large it is. It reports whether an a tag is present, and it looks up the DMARC record and tells you if the policy meets the precondition.
The mark certificate is not downloaded or validated here. Whether a certificate is required, and which kind, differs between mailbox providers.
What a passing check does not mean
A correct record is a request, not a switch. Each mailbox provider decides on its own whether to display a logo, based on its own rules about certificates, sender reputation and the message at hand. This tool reports whether the published pieces are in order; it cannot tell you that a logo is shown in any inbox.