Invalid location of tag

写表单时报错

• Invalid location of tag (th).
  th应当写在tr内

<tr><th>th>tr>

• Invalid location of tag (tr).
  tr应当写在table内

<table><tr>tr>table>

我修改后的代码

    <table>
    <caption>一个简单的加法器caption>
    <tr>
    <th>加数th>
    <th>th>
    <th>被加数th>
    tr>
    <tr>
    <td><input type="text" name="a"> td>
    <td>+td>
    <td><input type="text" name="b">td>
    <td><input type="submit" value="=">
    tr>
    table>