HTML Tidy is removing the DOC type declaration

I had picked source code of some sites ramdomly like www.nypost.com, facebook.com , www.bigpond.com. When i had provided these three htmlpages as input to tidy (command line with -m option), what i had noticed is that its removing 开发者_C百科the DOC Type declaration and modifying the source code.

Can anyone specify why its happening like that, is it a bug in Tidy or so.

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It appears to be configurable:

http://tidy.sourceforge.net/docs/quickref.html#doctype