Rob Beezer’s free, open authoring platform PreTeXt is what enables the fantastic HTML output for Active Calculus and so many other free, open textbooks.
As part of my work with the PreTeXt Advocacy Group (Sally Koutsoliotas, Jennifer Nordstrom, Bruce Yoshiwara, and me), I’m excited to announce that PreTeXt is featured in the December/January issue of MAA FOCUS; you can read our article on The Future of Textbooks at http://bit.ly/2Yy53vH.
The article highlights the new PreTeXt Catalog https://pretextbook.org/catalog.html,
which has over 50 PreTeXt projects — all publicly available, some still in development. In addition, the new PreTeXt Gallery https://pretextbook.org/gallery.html offers a smaller list of mature exemplars. All three texts from https://activecalculus.org are in the Catalog, and the single variable text is in the Gallery. There are many other fantastic texts there for you to consider.
Even more, if you’ve not used PreTeXt yourself, I hope you’ll consider using it for future or current projects.