Chris diagnosed my problem as MyST using my system font. I think it should not do that -- I have never seen this behavior on a non-MyST-book-theme site. My original report assumed that book-theme was intentionally using a monospace font. Original report below based on that mistaken understanding :) --- Of course, I think monospaced fonts are great for certain use cases like data entry or programming, where characters must be aligned in columns. But for prose, the default case in a book-theme project (i.e. where users have not explicitly delimited code/data with code fences), I would argue that monospace fonts reduce readability and are less accessible than variable space fonts. Prose written in monospace has an "alien" feel as well, it's just uncommon. And for a "book" theme, I believe it's rare to encounter a book written in fixed width typeface. I struggle to think of an argument in support of monospace prose, aside from the history of use in JupyterBook (and lack of affordances for "pinning" the theme), which is a valid argument, but I feel it doesn't overcome the arguments against. I personally struggle to read pages rendered using book-theme and without exception override the font in CSS immediately on starting a MyST project. Maybe an anecdote isn't useful :) Some references: * [APA "Accessible typography" myth \#4](https://apastyle.apa.org/style-grammar-guidelines/paper-format/accessibility/typography): "monospace fonts are never a good choice for readability" * Study: [Typeface features and legibility research](https://www.sciencedirect.com/science/article/pii/S0042698919301087). "readers with normal vision read Times Roman 5% faster than Courier Bold at moderate print sizes" (this didn't hold for readers with low vision). * Study: [A Comparison of Popular Online Fonts: Which is Best and When?](https://web.archive.org/web/20051125062841/http://psychology.wichita.edu/surl/usabilitynews/3S/font.htm). Courier was the 2nd-worst performer on reading time, outperformed significantly by Comic Sans (though Courier performed well on perceived legibility, I think that is not a meaningful metric for the prose use case). Courier scored low on subjective measures of personality, elegance, and fun, but scored well on "business-like". 🕴️