Comment by adrian_b

Have you verified that your browsers have correct settings for their default fonts, i.e. a real monospace font as the default for "monospace"?

Here the code is displayed with my default monospace font, as configured in browsers, so the formatting is fine.

There are only 2 possible reasons for the bad formatting: a bug in the CSS of the ACM site, which selects a bad font on certain computers or a bad configuration of your own browsers, where you have not selected appropriate default fonts.