Extracting interface data
Since the switch to the streaming download system in patch 4.0.1, the previously-used Interface Toolkit is no longer supported by Blizzard. Instead, the interface data and artwork can be extracted directly from the client (as of patch 4.0.6) by using the console commands ExportInterfaceFiles code and ExportInterfaceFiles art to extract the code and art, respectively. Once extracted, the files can be found in the BlizzardInterfaceCode directory where the WoW executable file can be found.
Interface scripts are best viewed with a Lua editor. To view .BLP files, use a BLP viewer like XnView
Viewing interface data on the web
An up-to-date github repository is available.