The problem is that to get the monospaced layout I have to use <PRE> ... </PRE> tags ("preformatted text").

Mudcat displays this wrongly: instead of simply reproducing what I typed in a monospace font, it also puts a space at the start of each line. This is enough to make most ABC software fail to recognize it as a tune.

So, delete a space at the start of each ABC line and it'll work fine.