I have no idea how it's supposed to work (or why the don't have a print CSS), but this works for pages written with Markdown:
- "Edit" the wiki page
- Copy all text
- Run a Markdown interpreter and pipe its result, e.g.:
kramdown > /tmp/github.html
- Paste your markdown
- Press Ctrl-D to finalize your input
- Open the generated HTML file and print it.