I'm building a site right now, so far I've painfully forced everything to be compliant and it looks pretty much the same across browsers. However, I'm starting to implement some third party/free javascripts which do things like add attributes (eg. order=2). I could work around this but it's a pain, and I'm starting to lose my principals of making sure everything is valid. Really, is there any point to working around something like this? I got the HTMLValidator plugin for firefox, and looking at most major sites (including this one, google, etc.), they aren't valid XHTML or HTML.
I have yet to experience an instance where the addition of a non-standard attribute has caused a rendering issue in any browser.
Don't try to work around those non-standard attributes. Validators are handy as tools to double check your code for unintentional mistakes, but as we all know, even fully valid xhtml will not always render consistently across browsers. There are many times when design decisions require us to use browser specific (and non-standard) hacks to achieve an effect. This is the life of a web developer as evidenced by the number of technology driving sites (google, yahoo, etc.) that do not validate.