I don't know about accessibility for all users including mobile, but I do know that people are currently hacking this up on their own, and those versions are likely to be less accessible than anything DW comes up with. For example, I see people who chose to set the background but not the foreground, not including a "skip" anchor for screenreaders, etc. This happens already. Better to have a DW version of it that uses current best practice, and encapsulates workarounds for platforms with potential difficulties, than have everyone hacking up things that are less good.

Implementation note: given that it would only have to be written once, centrally, the spoiler code can be far more complex than individuals usually use. That means we can include, say, iOS-specific stuff for iPhones/iPads (which can't highlight-to-read), and so on. I'm not a front-end web developer of any kind, let alone a mobile web specialist, but it seems to me that a simple javascript onClick which removes the special styling might do the trick (on iOS at least).

Another note re: colours... in the previous suggestion, people suggested black/black with a white border as a universally viewable scheme. As a reader I happen to prefer grey/grey, but that's irrelevant, because presumably it'd have a CSS class on it that could be styled via the usual means (i.e. DW styles or custom CSS). So remember that whatever colour combo we're talking about would just be a default, not a universal. (And if someone wanted to always see the spoilers, they could in theory set the CSS to make it readable rather than obfuscated.)