The referring page (none) should be valid XHTML 1.0, with two arguable exception: if it contains images, it will use the data-original attribute, which is not part of the XHTML spec. If it includes embedded YouTube clips, it will contain the allowfullscreen attribute, which is also not part of the spec. This link disables the attribute (you'll see a &validating=1), so the validator should report no errors.

That error is debatable. Yes, it's not allowed in XHTML 1.0, but newer HTML5 drafts allow it. I accept it because it's necessary for lazy loading of images. See here and here for a discussion. If you know a better solution to this problem, I'd be pleased to hear from you.

I check pages regularly after updates, but there's a chance that the W3 consortium validator will find other fault with it. Please let me know if you find it fails.