<html>
<head>
</head>
<body> <select name="sel" onchange="document.getElementById('txt').style.display=this.value">
<option value="block">display</option>
<option value="none">hide</option>
</select><br>
<input type="text" id="txt"><br>
</body>
</html>
<head>
</head>
<body> <select name="sel" onchange="document.getElementById('txt').style.display=this.value">
<option value="block">display</option>
<option value="none">hide</option>
</select><br>
<input type="text" id="txt"><br>
</body>
</html>
<select onChange="chkValue(this)"><option value="特定值">特定值<option><option value="其他值" selected>其他值<option></select>
<span id="tt" style="display:none"><input type="text" value=""></span>
function chkValue(obj)
{
if(obj.options[obj.selectedIndex].value == "特定值") document.getElementById("tt").style.display = ""
else document.getElementById("tt").style.display = "none"
}
<form action='1.html' method="get" >
<tr>
<td>
<select name = "select" onChange= "document.getElementById('in1').style.display = this.value">
<option value="block">hidden</option>
<option value="none">display</option>
</select>
<input type="text" id="in1" maxlength="5" >
</td>
</tr>
<br>
<tr>
<td>
<input type="password" name="password" onkeypress='return /^[0-9a-zA-Z]{0,8}$/.test(this.value)' style = "text-transform:uppercase">
<input type="text"
name="username"
onkeypress='return /^[0-9a-zA-Z]{0,4}$/.test(this.value)'
style="text-transform: uppercase">
</td>
</tr>
<br>
<tr>
<td>
<input type='button' name="submit" onclick="f1()">
</td>
</tr>
</form>
</body>