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  刚好前台区有人问了个类似的问题

  http://bbs.blueidea.com/thread-2839114-1-1.html
  
  没时间详解，自己先理解下
  http://www.w3.org/TR/CSS21/visudet.html#inline-non-replaced
  10.6.1 Inline, non-replaced elements
  The 'height' property doesn't apply. The height of the content area should be based on the font, but this specification does not specify how. A UA may, e.g., use the em-box or the maximum ascender and descender of the font. (The latter would ensure that glyphs with parts above or below the em-box still fall within the content area, but leads to differently sized boxes for different fonts; the former would ensure authors can control background styling relative to the 'line-height', but leads to glyphs painting outside their content area.)
  
  Note: level 3 of CSS will probably include a property to select which measure of the font is used for the content height.
  
  The vertical padding, border and margin of an inline, non-replaced box start at the top and bottom of the content area, not the 'line-height'. But only the 'line-height' is used when calculating the height of the line box.
  
  If more than one font is used (this could happen when glyphs are found in different fonts), the height of the content area is not defined by this specification. However, we suggest that the height is chosen such that the content area is just high enough for either (1) the em-boxes, or (2) the maximum ascenders and descenders, of all the fonts in the element. Note that this may be larger than any of the font sizes involved, depending on the baseline alignment of the fonts.
  
  简单的说就是：行内非替换元素的在文档流中的占位高度取决于行高“line-height”，
  而其垂直方向的padding, border and margin 是基于其内容的区域——这里即文本框（文字本身）
  
  而CSS2规范却在这里留下了悬疑：
  10.6.1 行内的非替换元素
  如果'top'，'bottom'，'margin-top'或'margin-bottom'为'auto'，它们的计算值为0。'height'属性并不适用，但是框的高度由'line-height'属性给出。
  10.6.2 ……
  来源 http://bbs.blueidea.com/thread-2839096-1-1.html

  总结：  实际就是PADDING的 上下高度并不在文档流的垂直方向占有宽度，导致其物理的占位位置关系无变化，所以，视觉上的位置感觉不出移动，但是其本身理论位置还是 增加了的，从A连接区域可以看出来。其垂直位置的占位高度由本身的行高控制和决定。

  水平位置 可以设置PADDING的。