For example:

If your audience understands you, then you've communicated effectively; and to do so, cater your writing to the specific audience you're addressing. Remember that excessive concern for exact grammar and spelling alone will not ensure clarity. [I'd include your thoughts regarding the tendency of many intellectually accomplished people to use poor grammar and spelling as a postscript. Although it is an insightful observation, it's tangential to the theme of your post].

Accommodate your vocabulary to your audience. Jargon and uncommon words that are well-suited to literate readers may be gibberish to others. Online, one can reasonably expect a moderately educated audience that will benefit from words that expresses a finer shade of meaning than small and simple vocabulary. Similarly, an audience of professionals will benefit from industry-specific jargon and more complex language.

In short: know your audience, and target your writing specifically to them.