Any of the HTML/XHTML validators should find exactly the same errors, so it shouldn't matter which one you use. The WDG validator tends to give slightly more friendly error messages.
Both the XHTML and CSS validators can work with files that are on either a web server or your computer:
- Files on the web server
- Just copy-and-paste the URL of the file into the form on the validator. It will download the file from that URL and check it.
- Files on your computer
- You first have to enter the right part of the validator site: follow the link “validate files on your computer”, “upload files”, or “by upload”). Then, click the “Browse” button and find the file you want to send.
All of the validators have options to include some warnings. It wouldn't hurt to leave the warnings on—they help you find things that you overlooked. On the other hand, when an assignment indicates that pages should validate, you don't have to worry about warnings, only real errors.