Unfortunately even if you pick nominally-equal-width glyphs, on the web you can still get screwed over by font substitution/fallback done by the browser.
First off, let me say it is awesome you're doing something like this.
But... I'm encountering a similar issue on both Chrome and Firefox on Leenucks. I guess it's possible you don't see the problem on your Mac because both these browsers use the OS to do font rendering.
Seems like maybe there are some universal half-width characters that can be used.
https://www.unicode.org/reports/tr11/