The iframe tag implements form submission without page refresh (no js required) --- parsing

Let’s look at a piece of code first (PHP example)

1. Form code (form.php)

<?php  
header("Content-type: text/html; charset=utf8");  
?>  
<iframe name="testIframeName" style="display:none;"></iframe>  
<form target="testIframeName" method="post" action="formAction.php">  
<input type="text" name="username"/>  
<input type="password" name="password"/>  
<input type="submit" value=" 提 交 " />  
</form>

2. Action code (formAction.php):

<?php  
echo "<script>alert(&#39;test....&#39;);</script>";  
?>

OK, the above code is very simple, but it realizes form submission without any problem. Refresh....Why is this?

Oh~ In fact, a very basic thing... is the attribute problem of the form tag... as shown in the picture:

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