demo: file: ///Users/qiaorui.chen/Site/css/helloworld.html
1. position: absolute; // absolute positioning, the reference target is a window, a scroll bar will be affected by the impact will be.
2. position: relative; // relative positioning, when the parent class using relative time, certain subclasses absolute reference becomes the parent.
3. Floating layout: float: left; // float page layout does not occupy the position; floating above a clear impact clear:both;