A major part of learning website design is testing yourself, instead of/as well as others testing you...
What i did, was i found a company that had a bad website, like this, and i tried to create my own version of the website, to see if i could do any better...
Just make a website and either look at it on your localhost, or use a free web host to begin with, and you will clearly see how much you know and how much you don't
About the validation thing, did you scroll down on that link?
Read the errors, they just interpreted it differently, like where height"" is shown to be false, because it has no number inside it, yet it still works fine on the browser.
Any real error can be seen by self-checking the website.