Helen Sword, Associate Professor of Academic Practice, The University of Auckland, New Zealand has written several books on the subject of academic writing. She doesn't focus specifically on web based writing, but the elements for making your writing clear apply to all media. You may want to take a few notes as you watch her presentation.
Watch this presentation she made a few years ago when her book, Stylish Academic Writing, was published. Her references to "Steven" are to Steven Pinker, author of The Sense of Style, the Thinking Person's Guide to Writing in the 21st Century (and many other books).
Though we are concentrating on writing for online readers, the way we tend to write is the way we've written for academic assignments and research papers. We use jargon and large words because we think they make us sound smarter, more knowledgeable. Now, here I am telling you to do the opposite because reading online is different, and that your readers are different.
Sword's research has involved academics and academic journals, and she brings some data to challenge what we think about academic writing. If what she says is apropos to writing for academic publication, then it's probably okay for online course material, too. Pay particular attention to the six 'Cs' she describes in this talk.
Sword also published The Writer's Diet, A Guide to Fit Prose. It's a small book that has a website with an online tool to evaluate the "fitness" of your copy. I find this tool more helpful for research and print publishing than online writing because it doesn't allow for any formatting. But it's worth checking out when you have the time.