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Which is what I do (add a space before and after). I didn't know you weren't supposed to put the spaces until someone pointed it out to me — suggested I was not an LLM because I added the spaces.

Makes me wonder if kerning is done correctly, if the em-dash would look like there were spaces before and after when there were not.

Not at all, no. Here's a few historical examples:

1903 edition of The Wizard of Oz — https://archive.org/details/newwizardofoz00baum/page/2/mode/...

A page from Life magazine, 1894 — https://archive.org/details/sim_life_1894-08-23_24_608/page/...

The Illustrated London News, 1843 — https://archive.org/details/illustrated-london-news-v002-184...

The em dash pretty much just joins the two glyphs together. It's supposed to look that way.

You can also use an em-dash with thin spaces (U+2009) or hair spaces (U+200A), but it doesn't work on HN—they just display as regular spaces.

The common guidance I've seen is en dash with spaces, em dash without.