> We've been there with CSS box model, DOM levels, (X)HTML(5), and CSS prefixes.
Other examples:
Content-Type. There were a few years where many sites were misconfigured to serve virtually every non-HTML file as text/plain. IE, deviating from the HTTP spec, always used its own algorithm to detect the actual file type and thus "worked", whereas Netscape (and later Mozilla) respected the header and would display "garbage".
Windows-1252 mojibake. Various Windows-based authoring tools simply used the native Windows code page 1252 to encode their text in HTML files, which were then claimed to be encoded as ASCII or ISO 8859-1. This led to documents that mostly looked correct but would have garbage sprinkled around due to added punctuation in Windows-1252 (primarily the distinct open/close quotes and em-dashes, if memory serves). IE of course handled these "correctly", i.e. interpreted ASCII or ISO 8859-1 to mean Windows-1252. I think this behavior is now specified by HTML5 under certain conditions.