实现方式一：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>汇率转换</title>
</head>
<select >
<option >123</option>
<option>456</option>
<option>789</option>
</select>
<button>互换</button>
<select >
<option >123</option>
<option>456</option>
<option>789</option>
</select>
数额：<input type="text" value="123">
<button>汇率转换</button>
<body>
<table border="1px" width="500px" cellpadding="3" cellspacing="3">
<tr bgcolor="red">
<td colspan="3" align="middle">按当前汇率兑换结果</td>
</tr>
<tr>
<th>1</th>
<th>1</th>
<th>1</th>
</tr>
<tr>
<th>1</th>
<th>1</th>
<th>1</th>
</tr>
</table>
</body>
</html>
实现方式二：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<style>
td{
width: 200px;
height: 30px;
text-align: center;
}
/*属性选择器*/
[colspan]{
background: red;
}
div{
margin: 0 0 5px 0;
}
select{
height: 35px;
}
button{
height: 33px;
}
input{
height: 30px;
}
</style>
</head>
<body>
<div>
<select>
<option>美元</option>
</select>
<button>互换</button>
<select>
<option>美元</option>
</select>
数额:
<input>
<button>按汇率换算</button>
</div>
<table border="1">
<tr>
<td colspan="3">按当前汇率换算结果</td>
</tr>
<tr>
<td>1</td>
<td>2</td>
<td>3</td>
</tr>
<tr>
<td>1</td>
<td>2</td>
<td>3</td>
</tr>
</table>
</body>
</html>