RelMeAuth is a way to identify yourself as your domain name while using third party OAuth providers to authenticate.
The goals of OpenID and RelMeAuth are similar. Both encourage you to sign in to a website using your own domain name. However, OpenID has been almost entirely abandoned, at least in part due to the complexities of the protocol. RelMeAuth is a simpler implementation of a similar goal, by leveraging other OAuth providers and behaviors that people are already accustomed to.
Yes, your rel="me" links do not need to be visible, but the html does need to be on your home page. You can hide the links with CSS, or include them as <link> tags in your html head.
Yes, the assumption is that people are willing to own their online identities in the form of a domain name. It is getting easier and easier to host content on your own domain name. See "Getting Started on the Indie Web" for some suggestions, including mapping your domain to a Tumblr blog, or signing up for a simple web hosting service like Dreamhost.
This service exists for websites to use if they don't want to implement OAuth code for each provider. As a user signing in to the site, you don't need to worry about whether the site is using indieauth.com or some other RelMeAuth service.
If an application is using IndieAuth.com as an auth service and IndieAuth.com is down, then logins to that website will not work. However this is just the same as if that site's own internal auth service is down had they implemented it themselves. Because of this potential risk, it is possible that some apps may wish to run their own instance of this software or implement RelMeAuth directly to avoid relying on a third party service.
We gladly welcome new providers! The goal is to support as many as possible so users are not reliant on any one in particular. Here is what you need to do to be supported by IndieAuth.com.
Google+ profiles are no longer supported, due to Google continuing to change the behavior and markup of profiles.
IndieAuth.com requests the minimum permissions from each OAuth provider. In some cases providers do not provide a scope that only verifies identity without also giving access to data such as public tweets.