Some of the icon tests are bogus. I deliberately don’t put any <link rel=icon> on my site, but have a favicon.ico containing (among other sizes) 16×16 and 32×32. Your tool complains about meta tags for 16×16 and 32×32 not being found in the HTML. Well, they’re not, but they’re not necessary, because I haven’t put anything in place that would disrupt the favicon.ico fallback. 192×192: … why? Won’t things gladly scale that 512×512 you want down? Manifest and other large icon sizes: this stuff isn’t relevant to all sites. And that’s a problem with these sorts of tools in general, they give scores tuned to a single usage profile which simply isn’t suitable in all cases. As with HTTP Observatory’s XSS stuff commented about elsewhere in this thread. What we need for tools like this is profiles that twiddle rankings. Things like “personal content site” which changes manifest and Apple/Safari/large icons to optional. As it stands, the weighting of this extra stuff is way off base—I get given an F for that section, when I honestly think it should get at least an A, when operating under my hypothetical “personal content site” profile.
Test 48 is bogus, the <body> start tag is optional.
Test 111, wanting initial-scale on the viewport meta tag, I’ve been casually searching for someone to confirm what it actually does, and if it’s still needed. Most indications suggest it was basically a workaround for an ancient iOS Safari rotation bug, but I’ve come across at least one person stating (without detail) that it still did something. Any chance you have Apple stuff and can investigate more as to whether it’s actually still at all useful?
Test 33, DMARC record formatting, looks bogus.