Some comments that would have helped me when writing my first extension:

• Extention
  ...
  Implementations of this interface should provide a method static Extension create() that returns
  an instance of the extension. This method is invoked by flexmark for extension classes set
  as Parser.EXTENSIONs using reflection.

• HtmlRenderer.HtmlRendererExtension#extend
  This method is called once when the builder for the output is created.
  It must configure the passed in builder as required by the extension.

• HtmlRenderer.Builder
  A builder is configured by adding factories for various aspects of the build process. The factories
  provide general information about the changes to be made and create the objects that do the
  actual work on the HTML tree.

At least, this is what I have understood.