I just noticed that when RSS Bandit finds an invalid RSS feed, it offers to fork off to Sam and Mark's RSS Validator.
If only Netscape Navigator and Internet Explorer had provided this feature back in 1994, I wonder how much better the world of HTML would have been.
-- Don Box
Actually, reaching way back into my memory, I seem to remember that we had, or almost had, a feature like this in IE4.
The story goes like this: starting with IE4 we switched over to the new advanced Trident rendering engine (mshtml.dll). We worked super hard to be compatible with all of NS3's quirky behavior. One thing that our architecture made it very hard for us to handle is the case where someone had both a <frameset> and <body> on the same page. Netscape handled this one way and it was really hard for us to do the same.
The answer that we came up with at the time was to have an "analyze" dialog that would look at the page and tell you why it wasn't doing what you wanted it to do. I don't think that anything really came of it and it was pulled at some point. The particular markup behavior that pushed us toward a solution like this disappeared from the web as well.
This wasn't a true "validate" dialog at such, but the push for pure wasn't there back in 97.
I'm not making any excuses here, but I think that everyone can agree that one of the reasons that IE was so successful at that time was because it could consume everything that was already out there on the net -- and that meant very careful duplication of all of netscapes bugs.