I think there's a much bigger danger in making accessibility standards mandatory than anyone has mentioned so far. That danger is that they might actually marginalize groups of users much more than happens now.

Take the WCAG 2.0 standard on contrast, for example. Essentially, it suggests that more contrast: good; less contrast: bad. Well, for some users, that's true. But for many (not all) dyslexics, for example, that's absolutely not true. Too much contrast actually exacerbates the problem. For this group of users, low contrast with colored backgrounds is much more preferable.

But that runs the risk of causing problems not just for those who need higher contrast, but also for those with certain forms of color-blindness. You could try to overcome that with different focus attributes, for example, but that risks adding clutter and artefacts which may hinder those with other accessibility issues.

I could go on ...

The point is that any choice of accessibility requirements will thus inevitably help with certain accessibility problems while exacerbating others. Yet, if someone who is now worse off were to complain about these new standards, s/he would be met with the response that the theme is "accessibility-compliant"!

So we'd have developers systematically producing themes that would be inaccessible to specific groups of would-be users, and yet this result would have been brought about in the name of accessibility!

Rules are great in certain circumstances. But, in a case like this, what's much more important is that developers (and site-creators) learn about the issues so that they can make informed choices. No doubt different developers will make different choices, and so we won't have a whole group of users who are effectively marginalized.