Another inevitability with a bug like this is that some will say Internet Explorer has unusually many bugs like this because Microsoft does not open its source code to public inspection. Whether they’re right with their assessment, I don’t know, but I doubt they can be right with the explanation. The slight difficulty of reading software like MSHTML without its C++ source code, especially if aided by symbol files, is nothing compared with the investment required for understanding the code.

From Geoff Chappell