Is the following valid HTML? What I am wondering about specifically is the location of the <form> tag.
<table>
<form>
<tr>
<td>
<input id="txt" type="text"></input>
</td>
<td>
<input id="txt" type="text"></input>
</td>
<td>
<input type="submit"></input>
</td>
</tr>
</form>
</table>
It's not valid. I'm guessing you want to remove the default padding of the form element. This is better done through CSS:
form {
padding:0;
margin:0;
}
or
<form style="padding:0;margin:0;">
Your approach used to be very common to remove the default form padding (before CSS became popular). Now you should put the form tags outside the table. Preferrably you shouldn't even use a table here, but that's not the question, so I'll let it slide. ;)
No this is not valid, TABLE can only contain TBODY, THEAD, TFOOT, CAPTION, COL, COLGROUP. If you are using XHTML then you can also put tr inside the table.
I recomment using the w3c validator http://validator.w3.org/. And no its not valid :-)
No. see: http://www.w3.org/TR/html4/struct/tables.html#edef-TABLE
<!ELEMENT TABLE - -
(CAPTION?, (COL*|COLGROUP*), THEAD?, TFOOT?, TBODY+)>
The <form> element can appear in all places where you can have block content. So <form> can go in <body>,<blockquote>,<noscript> (oddly enough also in <map>). Because block can also go in flow, it may also appear in <div>,<ins>,<del>,<dd>,<li>,<td>,<th>
not eaxctly an answer. Its not the right way but its mostly done to loose the empty lines.