It's been said that though the yachts get the glory, dinghies make the sailor. The same is true for HTML Newsletters. Websites get the glory. Newsletters make the developer.
With an online newsletter, you’re essentially building a mini-website. The hard part is that for the newsletter to keep a loyal audience and keep the brand firmly top-of-mind. The user-experience here reflects on the Brand dramatically. In other words, you can’t expect a user to believe your software solution is easy to use if you need to be a yacht master to navigate through it.
It also makes it difficult that the newsletter must have to stand up to transmission over a number of delivery systems through proxy and exchange servers, navigate around SPAM filters and email settings to arrive in one piece at the requested inbox.
Keep it clean, concise, simple, branded, intuitive and most of all, valuable. Keep one or two articles as a series to build continuity and have each article provide your audience with relevant, actionable insights into their everyday life. This is not the place to shout your value statement. Instead, tell your audience how to do it better or easier, and murmur your value statement under your breath.
Here are a couple of examples of HTML Newsletters that became online marketing work-horses:
Starr Tincup :: The Original Pickle

Originally spawned by Bret Starr as a promotional piece for his company, Starr Tincup, The Original Pickle was designed as an Onion-like online newsletter for marketing professionals. It's irreverent sense of humor, distressed but simple creative style and ease of use, made this newsletter generate a real return on investment for Starr Tincup.
Diligent Technologies :: The Diligent Factor
The Diligent Factor Newsletter was designed for Diligent Technologies as a customer retention tool. The newsletter served the purpose of keeping the company's customers up-to-date with industry best practices related to Diligent's value statement and apprised of any updates concerning Diligent or it's products. It also featured case studies, and thought-leadership downloads.
GridApp Systems :: The Database Monitor
The Database Monitor was built for GridApp Systems as a way of keeping top-of-mind with prospective clients. It's primary objective was one of thought-leadership.