I was wondering about this paragraph in the EPUB spec:

EPUB Content Documents that include spine-level scripting MUST utilize the progressive enhancement technique, which for the purposes of this specification has the following definition: when the document is rendered by a Reading System without scripting support or with scripting support disabled, the Top-level Content Document MUST retain its integrity, remaining consumable by the user without any information loss or other significant deterioration.

I indeed ran into the following Web page:

https://ciechanow.ski/cameras-and-lenses/

this is a (in my view) superbly done page on photography basics, their physics, etc, which has tons of interactive graphics examples that are essential for understanding the subject. Looking at the source all the graphics are done via scripts; as far as I could see via a canvas of some sort and using GL (I must admit I did not go into the details). It is all 'local', in the sense of there is no external reference to major frameworks: it is all a bunch of clever scripts and CSS. I.e., this content could be turned into EPUB 3.3 and it would be a nice example of what an educational EPUB would look like.

Except that... it violates the normative restriction above. In this very case, I just do not see how the author could fulfill the requirement above. This page, and the text, is inherently bound to the interactive possibilities in the graphics; the text is simply not understandable without it. As a consequence, and in spite of what was said in the previous paragraph, it does not seem to be possible to turn this into a compliant EPUB 3.3.

Is it necessary to keep the MUST here? Would it be enough to use a SHOULD?