Why do some websites prevent lion from sleeping automatically?

Hi, Everyone.
Sorry for yet another 'Lion won't sleep' thread. But I gotta ask this.

Like everybody else, I also noticed that my 2011 macbook pro does not sleep automatically. Since then, I tried everything to fix the problem. I disabled internet sharing, auto-synching option in the chrome browser...etc. But, I was still having random no-sleep problem.

Today I found out that my laptop does not sleep only when the browser is open with a couple of specific websites. These websites are Twitter and Steep & Cheap.

Can anyone tell me why these websites prevent mine from auto-sleeping? I was kind of glad that I was finally able to locate the problem but I have no idea why the issue has to do with specific websites.

Thank you.

Anyone????

I also found out that iTunes with no music playing and Google Cal are blocking auto-sleeping.

Can it be related? Those sites are constantly updated. For example, Twitter shows unopened twits, Steep and Cheap changes its sale items every 20-30 minutes...etc.

I have no idea. But I can say that with other websites open (for example, forums, espn...etc) mine goes into deep sleep without any problem.