Validating an email address
Let’s face it no one can really list every potential show stopping error where search engines are concerned.
The safest thing to do in that case is fix all your errors or as many as realistically possible.
The reality is as long as your site and pages display like they’re supposed to your visitors aren’t going to care whether or not the code behind your site is 100% valid.
Browsers won’t care much either as they’ll typically render much invalid code perfectly fine. Still there are good reasons to validate your code.
For example forgetting to close a tag early in your document could end up resulting in dozens, even hundreds of errors.
Fix those one or two errors and watch as many more errors are gone when you revalidate the page.
Note that I also left off the period after “This is a sentence” above, which violates the rules of grammar, but likely doesn’t cause any problems in communication.
Some will probably tell you that valid code will help your pages rank better. Other than show stopping errors (which validation can help you find) search engines really don’t care much about your code.
Validators will also issues warnings in addition to errors.
100% valid code means 0 errors, but it could include any number of warnings.
One advantage of HTML Tidy is using an extension you can check your pages directly in the browser without having to visit one of the validators.
Ultimately validation is simply a tool to help you code better.