Content Renderers in Tapestry are classes that operate on file types. Every file that is parsed by Tapestry has a renderer class matched to it by file type during the LoadSourceFiles step.
The HTML Renderer operates on any file with an extension of htm or html. This renderer was created to allow htm and html files to have Front Matter. If a file has a template defined in its Front Matter then this renderer will mutate it into a phtml file to be further rendered by the plates renderer.
Any input file that does not have an extension matching any other renderer will have the Default Renderer assigned to it. This renderer mutates files attached to it, marking them to be copied and not compiled.