Theming is not in line with flag module (and much harder)

Problem/Motivation

I replaced the follow/unfollow button with icons. With flag, I have two options to do so:

1. Add <span class="my-icon"></span> as flag/unflag label.
2. Add the same tag via template; the template knows about the two Flag/Unflag states

In flag_anon none of the options work for me.
1. The html is printed in plain text.
2. The template does not seem to know about the two flag states

How can we achieve this in flag_anon?

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes