让pre标签自动换行

pre标签会原样保留HTML内容的格式，可是如果宽度过大会把页面撑坏，这时候需要自动换行来帮忙：

/* Browser specific (not valid) styles to make preformatted text wrap */
pre {
  white-space: pre-wrap;       /* css-3 */
  white-space: -moz-pre-wrap;  /* Mozilla, since 1999 */
  white-space: -pre-wrap;      /* Opera 4-6 */
  white-space: -o-pre-wrap;    /* Opera 7 */
  *white-space: pre;       /* Internet Explorer 5.5+ */
  *word-wrap: break-word;       /* Internet Explorer 5.5+ */
}

pre {
    white-space: pre; /* CSS2 */
    white-space: -moz-pre-wrap; /* Mozilla*/
    white-space: -o-pre-wrap; /* Opera 7 */
    white-space: pre-wrap; /* CSS 2.1 */
    white-space: pre-line; /* CSS 3 (and 2.1 as well, actually) */
    word-wrap: break-word; /* IE */
}