http://htmlhelp.com/reference/html40/forms/form.html
It is true, because when I use:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Untitled Document</title>
</head>
<body>
<div>
<form action="index.php">
<input type="text" id="field1" name="field1" />
</form>
</div>
</body>
</html>
W3C validator show error:
Line 12, Column 46: document type does not allow element "input" here; missing one of "p", "h1", "h2", "h3", "h4", "h5", "h6", "div", "pre", "address", "fieldset", "ins", "del" start-tag.
and when:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Untitled Document</title>
</head>
<body>
<div>
<form action="index.php">
<div>
<input type="text" id="field1" name="field1" />
</div>
</form>
</div>
</body>
</html>
then: This Page Is Valid XHTML 1.0 Strict!