/* pre标签内tab替换为4个空白字符 */
pre{ 
    -moz-tab-size: 4; /* Firefox 4+ */ 
    -o-tab-size:  4; /* Opera 11.5 & 12.1 only */ 
    tab-size:   4; /* Chrome 21+, Safari 6.1+, Opera 15+ */ 
}