The outline-structure of your opml-file is represented in a foldertree that can be edited with a click of your mouse. You can easily add, edit and delete nodes and links.
#xml
RSS Extensions
How do you know what RSS extensions have been defined already, where they have been defined, and how you can avoid re-inventing what somebody else has created already?
The Atom Syndication Format
Official, hyperlinked spec
Google AJAXSLT
XSLT in JavaScript