目录来看一个简单的测试例子！特殊情况：None、True、False什么时候用is，什么时候用==？是不是刚学python没多久的童鞋们，就经常被 is 和 == 搞得晕头转向？明明看起来两个变量值都一样，用 is 判断却返回False？又
I have a block element that is positioned absolutely and some other elements on page that are positioned fixed. The effect is the block on top floats over the page which wo开发者_开发问答rks well.
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