One of the requirements we must be aware of and optimize for is accessibility.
* Non-ASCII characters: I do not know the deployment of support for Unicode strings in screen readers, but I imagine that it isn't as consistent as support for ASCII strings, especially for obscure characters (dingbats, emoji, etc.)
* Equations: I am told that MathML has a better story for accessibility than other forms of equations. Treating equations as illustrations reduces accessibility.
* Illustrations, diagrams: Explicitly requiring accessibility for illustrations (alternate descriptions) might make documents MORE accessible over ASCII art.
I think we should be EXTREMELY conservative about making accessibility of documents WORSE for ANY audience, and so explicit attention to accessibility is important.
The W3C recommendation "Web Content Accessibility Guidelines (WCAG) 2.0" is a must-read.
"Web Content Accessibility Guidelines (WCAG) are part of a series of Web accessibility guidelines published by the W3C's Web Accessibility Initiative. They consist of a set of guidelines for making content accessible, primarily for disabled users, but also for all user agents, including highly limited devices, such as mobile phones. The current version is 2.0."
Note that there are accessibility guidelines for HTML, PDF, SVG.
I don't think WCAG actually goes far enough for this. WCAG is focused on "how to make whatever content you have accessible", while we are focused on "whether to allow material which cannot be as easily made accessible".