I finished everything for the comment avatars yesterday including the fallback image for a broken link but, as this all applies to webmentions, I started wondering about normal comments.
Most commenting systems ask for your email address and use this to perform a Gravatar check. I deliberately didn't want to worry about email addresses, there's no real need for collecting them in this way and visitors shouldn't have to supply them just to leave a comment. It does, however, leave me without an obvious route for using other avatars.
Are there other options for pulling images from people's sites based purely on a provided URL?