It'd be impossible to prevent non-full URLs without also imposing extremely heavy restrictions on images, styling, and html. There are way too many ways to hide

It'd be impossible to prevent non-full URLs without also imposing extremely heavy restrictions on images, styling, and html. There are way too many ways to hide, spoof, or obfuscate an url. And even then, you'd need to somehow render the link in a way where the average user can understand it (which part of the URL is the domain and what implications it has).