if we avoid doctype in html it goes to quirks mode . what are the things will be affected
Browsers will assume that the site is using very old, outdated code and render the page according to old-style rules. Why are you asking? There’s no reason not to include a doctype these days. 
You also run the very high risk of causing IE to go to sh**s. Without a doctype, it will be reverted to IE5, and if you’ve had to code for that browser, blessings on your soul :).
As Ralph asked, I too am curious why you would even want to bother taking it out?