This page explains what
To use a validator, your codes must be on a webpage so you can submit the URL to the validator. That is the only way to input your codes. It is intended for checking webpage codes. To validate Webtv sig codes you would first have to copy them to a webpage to get a URL for them. IF you don't have a website for this, your Webtv pagebuilder could be used. Just create a page, add an item, click text, and paste the codes into the large text box, then publish the page to get the url.
You will find a validator in the source viewer and image extractor section of my tools page. This is the tool that will show you ALL of your html errors. It won't correct them for you, only tell you what is wrong. Sometimes it is wrong in it's analyses because of confusing tags and you have to look back at the previous tags for the error. Something like a missing quote mark is a common error that the validator will misinterpret because of not knowing what was intended to be quoted or where the quote was supposed to end. In cases like that, it may read the next tag as being in error instead of the one where the error actually occured. Using a validator to check your html pages is a good way to learn proper html form.