Although the new Webmaster Guidelines, which were recently modified by Google in January 2016, place an inherently significant degree of emphasis on the validity of the HTML source code that your web pages are built up on, the truth of the matter is that invalid HTML does not appear to affect page rank in itself. However, this certainly does not mean that validating your HTML using the relevant webmaster tools is of secondary importance in any way whatsoever.
Provided your HTML source code validates correctly by means of the W3C validator, you will find that your website is much easier to crawl from a search engine point of view, so you should always ensure that there are no broken links to be found within the internal structure of your pages meaning that everything works exactly as it should from the point of view of your end users.
Of course, with so many Webmasters inadvertently overlooking such issues, Google themselves say that to penalize a website design on the grounds of invalid HTML code would wreak havoc on the entire search result listings as we know them – which is not the intention they set out with when introducing this guideline.
So, in the simplest of terms, provided you check that your pages are carefully coded from the ground up, tested regularly and contain nothing but genuinely Validated HTML, you will be offering your visitors an online browsing experience that is pleasurable, easy to navigate and completely functional on every level without any annoying misdirects or unreachable landing pages.