既然thead已经有了1px的边框，为什么不会与tbody里面1px边框发生重叠

.table-bordered {
 border: 1px solid #ddd;/*整个表格设置边框*/
}
.table-bordered > thead > tr > th,
.table-bordered > tbody > tr > th,
.table-bordered > tfoot > tr > th,
.table-bordered > thead > tr > td,
.table-bordered > tbody > tr > td,
.table-bordered > tfoot > tr > td {
 border: 1px solid #ddd; /*每个单元格设置边框*/
}
.table-bordered > thead > tr > th,
.table-bordered > thead > tr > td {
 border-bottom-width: 2px;/*表头底部边框*/
}