若想对标准输入、输出、错误输出等进行重定向(redirect)时，可以使用IO#reopen:

line-height只对内联元素有效，对于块元素设置其实是作用在其内部的内联元素上。

水平七属性中margin-left、margin-right、width的值可以设置为auto，其他四个属性不能设置为auto。

水平七属性相加的值总是等于其父元素的width值。

margin的值可以取负值，padding和width不能取负值。

一行中，不同内联元素的内联框位置并不一定相同，根据line-hight、font-size计算，并受vertical-align影响。以下公式在vertical-align基于baseline的计算方法：

内联框的上边框位置：font-size - (font-size - line-height) / 2

内联框的下边框位置：font-size + (font-size - line-height) / 2

行间距等于该行中所有内联元素的内联框(inline-box)的最高位置减最低位置。

当将margin应用于内联元素时，margin的上下边距将不会影响到行高--产生的margin是存在的，不过因其是透明的故不能看得到，左右边距则会影响内联元素的左右间距。

如果将一个将粗的border应用于内联元素上时，行高也不会发生变化，border将会覆盖其他元素的显示。

能够改变全文本的行间距的CSS属性只有:

1. line-height
2. font-size
3. vertical-align

Absolute Positioning:
Understanding absolute positioning means understanding the concept of the document flow.
Absolute positioning provides the ability not only to move an element anywhere within the page, but also to remove it from the document flow. An absolutely positioned block no longer influences other elements within the document, and the linear flow continues as if that block doesn't exist.
Relative positioning:
Relative positioning, on the other hand, will not remove an element from the document flow. Based on the element's starting position, a relative position will offset it and then effectively leave a hole behind that other elements must negotiate, as if the element were still positioned there.
A relatively positioned element stays within the document flow; its originating position continues to affect other elements, whereas its new position is ignored by the document flow.
Relative positioning is mostly useful for offsetting elements; an element with a starting position inside a traditional grid may be easily moved outside the grid structure, and elements can be finely adjusted when other positioning options aren't possible. For example, Savarese employs relative positioning to move the dragon at the bottom of the design from a starting point below the white content area, to its final spot at the left of the footer area. Using absolute positioning to position elements near the bottom of a design is much more difficult than positioning them near the top, so this instance of relative positioning makes the adjustment easier.

Exploiting the fact that Internet Explorer lacks support for fixed positioning and for child selectors, a series of rules were created to deliver the optimal design to browsers that comply with the rules of CSS, and to deliver the alternate design to Internet Explorer: