tedster - 6:15 pm on Mar 4, 2012 (gmt 0)
One reason that Google sometimes offers code snippets that don't validate is that the kind of validation "problems" that do exist don't really matter very much. They don't affect SEO, and they don't affect a browser's ability to display the page properly.
HTML is an extremely forgiving technology - by design. It's not like C++ or any hardcore programming language. In cases like this, I usually don't worry about perfect validation - life's just too short!
Knowing when a validation shortcoming really matters and when you can safely let it slide can bring a lot of ease into your online life. No alt text is one of those "doesn't matter much" areas. Invented tags like <g:plusone> also don't really matter - browsers will just not worry about them.