We have all seen the W3C validator. Most SEO’s have used it. I used it in the beginning to help sell services and while I was at various SEO firms it was a crutch that we relied on to sell services. Now I stay away from it like the plague. When I see an SEO mention WC3 issues I first ask why they are using it. If you look at any site out there you will see that the many browsers no matter what will still render your site.
If an SEO tells you that you have major errors using W3C and that is causing you not to rank. Run from that SEO. Do not trust them. The page still loads, and you will be okay. You have to look at each error and warning in W3C and see if there is any reason that it will impact your site. Looking at some of the biggest sites in the planet they have a few hundred errors. If you have a few thousand, then that might be something to think about. You need to weigh each one. Go look at Target, Walmart, your competitors and you will see how many errors they have. If they have more than you, you should not worry that much about your site but the SEO that you are talking to.
If you are trying to do a snake oil type of sale then you are a scum bag. Yeah I said it, you are relying on a parlor trick that has no basis for rankings and you know it. If a site has warnings and errors check them to see what they are actually doing. Don’t be the scumbag that just says the site has 30 warnings and 20 errors and that is why the site is not ranking at all. Don’t be the parlor trick player that has no business being in this industry. If you do not know what W3C is actually used for, that is okay. You do not have to use W3C to do your audit.
Take W3C with a grain of salt. Check each warning and if anyone tells you that your W3C errors and warnings are affecting your SEO, ask why. Dig down the rabbit hole and understand why. If it is just a missing > or something like that but the browser has figured out how to load the site, you are okay.