Content handling mechanisms
The task of detecting and handling various file types and encoding schemes is one of the most hairy and broken mechanisms in modern web browsers. This situation stems from the fact that for a longer while, virtually all browser vendors were trying to both ensure backward compatibility with HTTP/0.9 servers (the protocol included absolutely no metadata describing any of the content returned to clients), and compensate for incorrectly configured HTTP/1.x servers that would return HTML documents with nonsensical Content-Type values, or unspecified character sets. In fact, having as many content detection hacks as possible would be perceived as a competitive advantage: the user would not care whose fault it was, if example.com rendered correctly in Internet Explorer, but not open in Netscape browser - Internet Explorer would be the winner.
As a result, each browser accumulated a unique and very poorly documented set of obscure content sniffing quirks that - because of no pressure on site owners to correct the underlying configuration errors - are now required to keep compatibility with existing content, or at least appear to be risky to remove or tamper with.
