Well, the Transitional DTD was meant to be just that... "transitional". That transition period has long passed, so you shouldn't be using that one any more. :)
maybe they just don't like Transitional, period. (What the ### business is it of theirs?)
Umm... well... since they (the browser) is the one rendering the page, I'd say it's their business. I noticed that on WebmasterWorld the tooltip on the DOCTYPE says
Quirky doctype. Expected "<!DOCTYPE html>".
So perhaps the warning is really just meant as "this DTD will trigger quirks mode, and as such is less cross-browser friendly"? Of course, WebmasterWorld is using a DOCTYPE that will trigger quirks mode (*shakes head*). Just a guess, though. I don't think the "Expected" statement necessarily means that they ONLY expected an HTML5 doctype. It might just mean they expected something that would trigger standards mode instead?