I want to show some content in a HTML widget only to users with a certain browser.

I have a <div style='display:none;'> which I turn "inline" via javascript when a certain browser is used. 

My code works perfectly in an independent file, but not in the widget. The toggling just not works. If the <div> is defined initially as hidden it always remains hidden, if I define it as visible, then I cannot hide it...

Does anybody know how I can make this work in a widget?