Alpha Idea

// insertcell(3)在第三行后加一行,-1代表最后一行
tr = document.getelementbyid("tablename").insertrow(-1);
td = tr.insertcell(-1);//加列
td.align="right";
td.colspan=3;  // s要大写
td.innerhtml = "表格内容";

table = document.getelementbyid("tablename");
tr  = document.createelement("tr")
td = document.createelement("td");
td.innerhtml = "表格内容";
tr.appendchild(td);
table.appendchild(tr);

for (i = table.rows.length-1; i >= 0; i--)
      ta.deleterow(i);