The current content for education is good, but is definitely bandwidth-heavy and is tough to maintain. But dropping to plain text will force us let go a few things that otherwise make learning more effective.
I think HTML (or a WYSIWYG style editors - that seem like plain text, but can be powerful with images, videos, animations when required) also does the same thing like plain text,
- it is always compatible (I give it to you that it takes effort to run hifi stuff on browsers, but still better than plain text).
- is easy to mix and match
- is easy to maintain (thanks to many editors)
- is light weight (not compared with plain text of course)
- is forward compatible (not possible when all browsers decided to not support HTML in its current state).
I appreciate the thought behind bringing this up. I think writing something in plain English, which can then be turned into some super cool learning material that runs everywhere would be awesome. It helps both in solving the issues mentioned in the article at the same time keeps learning effective.