Look what the google says
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Our programme policies do not permit the modification of the AdSense code. Your publisher account offers a number of options when generating the ad code, that we hope will allow you to create an ad layout that fits in with your site.
Please paste the AdSense ad code into your web pages without making any modifications.
Again, please note that modification of the AdSense code is a violation of the AdSense programme policies. For example, publishers may not:
Alter the height or width of an ad unit from the standard options
Implement the AdSense ad code in a "floating box script" or in a way that otherwise covers content
Use any means to force the display of more than three AdSense ad units on a page
Manipulate the ad targeting using hidden keywords,
IFRAMEs or any other method
Distribute ads in emails or software
https://www.google.com/adsense/support/bin/answer.py?hl=en-uk&answer=9997