Not a bug
BYOND Version:507
Operating System:Windows 8
Web Browser:Chrome 39.0.2171.95
Applies to:Webclient
Status: Not a bug

This is not a bug. It may be an incorrect use of syntax or a limitation in the software. For further discussion on the matter, please consult the BYOND forums.
Descriptive Problem Summary:
Using the web-client, a 2nd map element (Interface element not a map file) won't work at all. Basically, I output objects on a 2nd map and animate them.

Instead, on the 2nd map, it just shows a black map.

Numbered Steps to Reproduce Problem:
Create a 2nd map in your game's interface and output animated objects to it.

Expected Results:
Shows the 2nd map with the outputted objects in it.
Actual Results:
A black map.

Does the problem occur:
The problem just occurs in the web-client.
When does the problem NOT occur?
When running the game on dreamseaker.

Lummox JR resolved issue (Not a bug)
The fact that secondary maps aren't supported for the webclient is documented. I think it might be possible to rig something up as a future enhancement.