> The second, you could have easily addressed by proposing a markup if it were easy to do so. The fact that you haven't, and aren't even aware why people think it's hard, means you're again coming from an uninformed position.

I didn't propose any specific markup because the specific syntax isn't the problem. The problem is that the predominant markup language for mathematical documents is LaTeX, which is a bit too presentation-oriented rather than semantic/structure-oriented for it to be really easy to use it to produce documents that can be interactively folded (especially when the output file format is almost always PDF). Something that used eg. MathML and an HTML/XML based overall document structure can trivially encode all the information necessary for rich interactive reading, and proposing specific tag or class names to indicate what lines of a proof should be collapsed by default only invites bikeshedding.

I think you're quite unjustified in accusing me of being so thoroughly uninformed.

I'm not surprised. Virtually all "designers" think low contrast text is better, despite high contrast being preferred on paper. They deliberately reduce contrast by setting the text color to gray. Even Firefox's Reader View, which otherwise fixes most mistakes of designers, sets the text to gray unless you override it in userContent.css. Desktop GUIs often have low contrast text too, which is also difficult to fix. Unless somebody somebody knows how to do this their expensive monitor will be wasted.

The real reason people don't do this is simple, it would require an inhumane amount of work and details which will likely be wrong themselves and will for sure be to boring to be checked. Most science is wrong in some aspects, sometime it is better to have a meaningful intuition that can be understood by other expert (and maybe rejected)