The web browser is an excellent example; the problem started, however, before NS was even NS -- the initial versions of Mosiac offered no clues to the user that the web page was malformed, and that trend has continued since. Is this necessarily a good idea? It's a big if, of course, but I would suspect that if my browser told me every time I encounted a page that didn't meet W3C standards, I'd be very very tired of getting that warning nearly all the time. From a UI stand-point, some would argue that not informing the user of a bad layout in a web page is a good thing (You should only tell the user that there's a serious error if they are supposed to take action to do something about it). But here, the problem was that there was no way of using the browser alone to determine bad makeup.
in reply to Larry vs. Joel vs. Ovid
This lead to the development of the first generation of WYSIWYG tools that did produce bad layout as well. These became popular, so the second generation of browsers decided to make sure that they could accept those. Begin vicious cycle.
Nowadays, we're still stuck in the HTML problem, and I don't think that will ever be cured. But for so-called web application or interprocess communications, we have XML. There is *no* reason not to be able to write valid XML given the large number of tools, free, commercial, compiled, interpreted, or otherwise, out there; nor is there any reason not to read in only strict XML. However, the advantage of XML as a format is that one can include excess or alternative data to the format, which client A might not completely understand but can still produce an intended result, while client B can take advantage of. But the key point here is that the XML *must* be well-formed. If one gets bad XML, what should the error be the user? Hopefully, those that have worked with HTML over the years understand that malformed XML is potentally more dangerous than malformed HTML, and will take steps to tell the user of such.
Dr. Michael K. Neylon - firstname.lastname@example.org
"You've left the lens cap of your mind on again, Pinky" - The Brain
"I can see my house from here!"
It's not what you know, but knowing how to find it if you don't know that's important