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][1].  
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!

  [1]: https://stackoverflow.com/questions/954327/hidden-features-of-html