Adam Goucher wrote a wonderful series of blog posts back in 2009 that talk about this stuff, in the context of writing a Selenium IDE plugin (which the formatters are):
- The Selenium-IDE 1.x plugin API (Part 1) - Build and Preferences
- The Selenium-IDE 1.x plugin API (Part 2) - The Toolbar
- The Selenium-IDE 1.x plugin API (Part 3) - The Sidebar:
- The Selenium-IDE 1.x plugin API (Part 4) - Extending the Selenium API
- The Selenium-IDE 1.x plugin API (Part 5) - Publishing Updates
- The Selenium-IDE 1.x plugin API (Part 6) - Formatters
- The Selenium-IDE 1.x plugin API (Part 7) - Packaging Nuances
- The Selenium-IDE 1.x plugin API (Part 8) - Localization
- The Selenium-IDE 1.x plugin API (Part 9) - Secure Password Storage
- The Selenium-IDE 1.x plugin API (Part 10) - Registering your plugin
There's also some useful information at the IDE release notes on formatter plugins , and the old Wiki has a page that describes how to write a formatter (be forewarned, it's O-L-D).
The Selenium HQ downloads page has examples of several formatter plugins that you might find helpful:
- Play! Framework: A plugin for Selenium-IDE that adds support for the simplified markup used by the Play! framework.
- Grails Formatters: Adds Grails Formatters.