It lets you open any of the maps included with Warcraft II BNE (including campaign maps) or load maps off your computer.http://jcfields.gitlab.io/warcraft2-map-editor/
My intention is ultimately to turn it into a full map editor, but right now, it only opens maps. It can also save map images using "Save Map as Image" under the "Editor" tab. It'll probably be a while before I implement full editing functionality, but it's fun to work on a little here and there.
Works well in Firefox and Chrome. Works in Safari but loads very slowly.
The source code
is available for anyone who's curious. Hopefully someone else finds it interesting or useful.
Edit 6/26: I took a break from it but finally implemented saving, so you can use it to modify unit data, upgrades, and so forth.
Edit 6/30: Implemented restriction data (i.e., the ALOW chunk).
Edit 7/11: Implemented unit placement/deletion.