Right and correct markup in html forms - html

Question

Once upon a time in HTML
I was looking W3Fools and find this paragraph, related to the errors in W3Schools:
<form>
First name: <input type="text" name="firstname" /><br />
Last name: <input type="text" name="lastname" />
</form>
This code is wrong. Non-block-level elements (such as <input>) are
not valid directly inside <form> tags until HTML5.
Well, since I always format that way I said to myself "WAIT, I was wrong all this time". And go to validate it to check if it's true. Efectively, this code don't pass the validation.
Then, searching for the correct markup, I found several examples from more reliable sources, but none of the following examples passes the validator.
The bad, the good and the ugly
1. W3Schools
<form>
First name: <input type="text" name="firstname"><br>
Last name: <input type="text" name="lastname">
</form>
2. Mozilla Network Developer
In "Examples" section, I have found this:
A common use-case senario
<!-- A common form that includes input tags -->
<form action="getform.php" method="get">
First name: <input type="text" name="first_name" /><br />
Last name: <input type="text" name="last_name" /><br />
E-mail: <input type="email" name="user_email" /><br />
<input type="submit" value="Submit" />
</form>
3. W3C Wiki
<form action="http://www.google.com/search" method="get">
<label>Google: <input type="search" name="q"></label>
<input type="submit" value="Search...">
</form>
A Fistful of Codes
More details:
I'm trying to validate the code as HTML fragment, both in HTML 4.01 and XHTML 1.0.
The recurring error I have found is:
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
The question
What is the correct and valid markup I should use to make a form?
Thanks in advance.
Edit:
A similar question was answered redirecting the OP to the form section of HTML4 specification. They provided this code:
<FORM action="http://somesite.com/prog/adduser" method="post">
<P>
<!--...stuff...-->
<INPUT type="submit" value="Send"> <INPUT type="reset">
</P>
</FORM>
First, it isn't the same code of their wiki. But nevermind, from I know, the wiki it isn't 100% accurate.... BUT:
They fix the validation error by enclose all tag in a <p>. But forms are not paragraphs, it isn't? This code pass validation, but I think that's not very semantic...
Expert answers you giving to me (thanks!), for the most part, are to enclose the inner tags in a <fieldset>, but if I only need one fieldset (for example, a form with only one field or self-related fields), is this correct? Might that not be superflous code (because the fieldset tags are supposed to enclose similar fields)?

Answer 1

A fieldset is a block level element that is commonly used in forms. Therefore you could use a fieldset or another block level element like a div.
<form action="http://www.google.com/search" method="get">
<fieldset><legend>My form</legend>
<label>Google: <input type="text" name="q"></label>
<input type="submit" value="Search...">
</fieldset>
</form>
or
<form action="http://www.google.com/search" method="get">
<div>
<label>Google: <input type="text" name="q"></label>
<input type="submit" value="Search...">
</div>
</form>
This probably rarely gets flagged as non valid markup because people tend to a lot of divs to help style forms nicely.

Answer 2

I tend to agree with a few of the others in that FieldSet is the best way to group form items together, especially as it really helps with things like screen readers.
I tend to use the following:
<form action="http://www.google.com/search" method="get">
<fieldset>
<legend style="display: none;"></legend>
<label for="q">Google:</label>
<input type="text" name="q">
<input type="submit" value="Search...">
</fieldset>
</form>
I normally don't include the legend, but if it is required for validation, it doesn't necessarily have to be visible ;)

Answer 3

This code is wrong. Non-block-level elements (such as <input>) are not valid directly inside <form> tags until HTML5.
That's not entirely true. They are valid in Transitional DTDs but not Strict DTDs.
What is the correct and valid markup I should use to make a form?
It depends on the form.
For the examples given, the W3C version is the best. Having a block container in a form doesn't matter (unless it adds useful semantics, or sensible grouping of separate groups of controls). <label> elements are excellent.

Answer 4

I tend to group fields and labels inside the <fieldset> tag, which also sounds very semantical to me.