It seems, one can't have a mapping, which would end with a minus sign ('-'). A single minus sign works just fine, though.

To overcome that, a user needs to add one more minus to the mapping. E.g., to map the two key sequence a-, one should create a mapping for a-- instead.

For example, despite two minuses in a mapping below, bookmarks will be opened immediately on the first key press:

keymap -- BOOKMARK

However, if we put 3 minuses in the very same mapping, like this:

keymap --- BOOKMARK

...bookmarks will only be shown after two consecutive minuses.

Probably, w3m treats one of these minuses as a junction between keys (as it would be used in something like C-a), and therefore doesn't add it to an actual mapping. Some sort of a guard could probably solve that :)