To bring in more use cases I'm thinking w3schools.com style:

MozXBL1:
Say you have 3 editable TEXTAREA's (data, CSS, XBL) plus the result.

feel welcome to implement :-)

Ruud



PS:
so far would be great, but dreaming on:
- view as 'plain' XML (gives free syntax coloring, Well-Formedness
check, collapsability) instead of editable (3 switches)
- validate with the unofficial XBL1 RNG Schema.
- DOM inspector area
- snapshot capability