Google has said many times that poor or invalid HTML validation doesn’t impact rankings. Of course, web sites that don’t work, a search engine cannot crawl. But if your site works but doesn’t validate with HTML validation tools does not impact crawling, indexing, or ranking.

Google’s John Mueller said on Google+ this week that poor HTML validation may impact other SEO related aspects, outside of ranking. John said it can impact “meta-tags, structured data, or link-elements (for example, if your hreflang markup isn’t being picked up properly).”

He even gave an example:

So keep this all in mind, as schema and these other implementations make more of an impact on your CTR.

Forum discussion at Google+.