HTML validation and Google's web-search ... validation isn't necessary for crawling, indexing, or ranking, as we've said  many times in the past. However,
if you have problems with meta-tags, structured data, or link-elements (for example, if your hreflang markup isn't being picked up properly), then using a HTML validator can sometimes point you at problems in your markup.
In a recent example, a site was accidentally including a banner in the HEAD of a page. That might look ok in a browser, but it breaks the markup in the HEAD, causing us to potentially ignore the rest of it. A HTML validator can catch that for you. While validators aren't perfect, they will help you get closer to clean markup, which will make it easier to automatically extract your site's meta-data.
Personally, I use the https://validator.w3.org/
when I need one, there are lots of other options too.
 even together with others :-) -- https://www.youtube.com/watch?v=2XlKn6I9rSc