This is one of those ranking factors that is largely shrouded in myth, because a common perception is that your web page must be completely free of HTML errors.
In the real world, unless you are a super geek at writing code and fixing bugs, it’s actually fairly common to have at least a few errors on any given page. Most of the time the end user sees no difference in the resulting page being displayed.
It’s really only when the resulting page has so many functional or display issues that the user experience is negatively affected that search engines may consider your page to be less worthy of rank. And after all, that is actually what really counts: How well your user can interact with your web page without that interaction being obstructed by code errors.