The zero width characters might be leftovers from the transcription process, marking disagreements or known spelling errors or something.
If you click into one of the books on that website, you can see options to mark stuff as "ex" or "unc", which explains the weird markup (<span class="ex">er</span>).
The markup flags up abbreviations. I'm not quite clear on the distinctions between the different tags but it looks like "ex" is for expansions/excluded letters and "sl" is something to do with superscript letters, or thereabouts.
Screenshot taken with addon: https://i.imgur.com/rgG0tMb.png Screenshot taken without addon: https://i.imgur.com/BV3sxqP.png
The zero length character is also at the end of the title in this hacker news entry.