Currently the background note, private/logged contact note, and presumably the 1:1 email content (haven't tested that) text fields support CSS/HTML tags — albeit inconsistently (e.g. contact text fields support unencoded line breaks, whereas the background note does not).
Such notes are key to a core workflow of NationBuilder users. Text styles & formatting are helpful for drawing CPU readers' eye to key points in messages.
But adding full HTML/CSS tags to text fields is inaccessible to those who don't possess the technical know-how, and still cumbersome for those who do. One lightweight option that wouldn't involve adding a full rich-text editor might be adding support for markdown (or Markdown Here?), which's gained increased exposure through many popular tools like Slack, Trello, and Discourse. Just a thought :)