I've not much to say on the thermal printer part of this but the extensions they did to markdown are great. They had me double taking for a few seconds thinking they might be real markdown because they make so much sense.
[align=center] Center-align the following text (also left, right)
[qr=https://...] Generate and print a QR code
Deprecating align in html was a mistake.Since align is only deprecated and not removed, and Markdown is a superset of HTML (at least for CommonMark and GFM), it would be valid markdown to just use <div align=center> to center text (not like there's such a thing as invalid Markdown)
It’s funny, after all the work that was done to decouple content from presentation, 90% of the markup I’ve seen in every codebase this decade is using Styled Components anyway, which commingles them in the source code anyway.