Web design: how to check for errors

Self-tutoring about web design: the tutor mentions a way to check a site for errors.

Sometimes a webmaster might get emails telling them about errors on a site they manage. In my experience, these emails can seem random and are typically not to be trusted.

If one wants to check their own website for errors, it can be done. I just go to validator.w3.org. From there one can enter a web address, upload a file to verify, or perhaps even directly enter web code.