Draw a horizontal guide around 450px from the topborder, we will use it as a reference guide to add a shade on top of the page.
用顶边下来450 px处画一条水平参考线，我们将要用它给顶部添加阴影作为参考。
2
Left - Increase the generated box's top margin so that the topborder edge is below the bottom margin edge of any left float boxes generated from previous source elements.
left -增加所生成框的上边框，以便上border edge处于由之前的源元素所生成的任一左浮动框的底margin edge之下。
3
Right - Increase the generated box's top margin so that the topborder edge is below the bottom margin edge of any right float boxes generated from previous source elements.
right -增加所生成框的上边框，以便上border edge处于由之前的源元素所生成的任一右浮动框的底margin edge之下。