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I think your pretty close on the mark Brash. Once things are packaged a little differently, it will be easier to ensure compliance. As it is right now, each modules output is different and most do not comply with XHTML. You can get luck on CSS, like I did. I'm not willing to dump a module because it's not compliant, I rely to heavily on it.
I would however like to see more focus on this from other module developers in the future, and possibly have a 'clean up' crew on the module submission team to either clean and conform it themselves, or reject those that do not conform, or offer a conditional release of some sort.
It could go a few different ways. Good point, good post. Looking forward to seeing more in this area.