the benefit to most Mailman users would be
Imagine the user experience when reading/writing a github issue and/or getting an email about it. There is some formatting, even if limited, such as code blocks. Users appreciate that in technical code discussions, and so it's a benefit. They may think "Why can't I use italics?" Allow basic formatting. Remove colors, font sizing, for example.
construction of the DOM and a pretty capable CSS implementation
Basic html tags here are bold, italics, hyperlinks, code blocks... if these were allowed through a new html sanitizer layer, and could then be presented to the reader on Hyperkitty, I don't see why that would require anything advanced in terms of "construction of the DOM and a pretty capable CSS implementation". Just send those html tags to the browser, as-is. Perhaps nothing needs to change in terms of CSS or DOM.