Bringing attention to errors and attempting to recover from errors aren't mutually exclusive. I would much prefer that poorly formed pages cause the address ba

Bringing attention to errors and attempting to recover from errors aren't mutually exclusive. I would much prefer that poorly formed pages cause the address bar to change in some visually distinctive way, similar to how modern browsers make part of the address bar green for pages retrieved over HTTPS. Another option would be to display an error page with a button that says something to the effect of "try and guess what the lazy developer wanted". An inability to guarantee valid output hints that inputs aren't being properly analyzed and sanitized, with terrible security consequences. Both developers and users of pages should be made aware when these errors occur. If the error messages are too intrusive, adoption of browsers displaying them would of course suffer greatly.