Many of the Hidden Features question brings out hidden gems and hence adds good view value to it.
Check for example the Hidden Features of HTML.
I have been working in HTML since 1998. Still I had no idea about the first 3 answered features.
I am willing to bet that most of you did not know at least one of them, please comment honestly!
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replaced http://stackoverflow.com/ with https://stackoverflow.com/
Binoj Antony