@bender@twtxt.net Good point, according to Metadata ext: #avatar:
avatar
This specifies the URL for the author’s or feed’s avatar, so it can be displayed along twts, e.g. next to the author. The avatar image is typically in JPEG, PNG or WebP format. Different clients prefer different ratios, so there is no strict rule to follow for feed authors. Often avatars are square.
Anyway, looks like at least a few in the community are in agreement that we should maybe amend the spec and any graphical clients like yarnd to support relative URI(s).