Thank you Alexander! I've got it working now!
I don't know if this is worth mentioning or not, but I initially got this error when I clicked the Bake button:
Traceback (most recent call last):
File "/Users/user/blend4web_sdk_free/addons/blend4web/anim_baker.py", line 90, in execute
File "/Users/user/blend4web_sdk_free/addons/blend4web/anim_baker.py", line 207, in process_action
armobj.animation_data.action = action
AttributeError: bpy_struct: attribute "action" from "AnimData" is read-only
location: <unknown location>:-1
I think this is because I downloaded the .blend file from this post onto a different computer than the one it was created on, and had a file permissions problem. I was able to resolve this by looking at the file info and granting Read & Write permission to all users. I don't know if this will help anyone having a similar issue, but just in case…
One thing I am curious about: The option to Use Blender's Native Baker
… for what kinds of animations would it be better to use the native baker instead of the B4W one? Or should I just always try the B4W one first and only resort to the native one as a fallback if I have a problem?