Is it necessary to write HEAD, BODY and HTML tags?
Is it necessary to write <html> , <head> and <body> tags?
For example, I can make such a page:
<!DOCTYPE html>
<meta http-equiv="Content-type" content="text/html; charset=utf-8">
<title>Page Title</title>
<link rel="stylesheet" type="text/css" href="css/reset.css">
<script src="js/head_script.js"></script><!-- this script will be in head //-->
<div>Some html</div> <!-- here body starts //-->
<script src="js/body_script.js"></script>
And Firebug correctly separates head and body: 
W3C Validation says it's valid.
But I rarely see this practice on the web.
Are there any reason to write these tags?
Omitting the html , head , and body tags is certainly allowed by the HTML specs. The underlying reason is that browsers have always sought to be consistent with existing web pages, and the very early versions of HTML didn't define those elements. When HTML 2.0 first did, it was done in a way that the tags would be inferred when missing.
I often find it convenient to omit the tags when prototyping and especially when writing test cases as it helps keep the mark-up focused on the test in question. The inference process should create the elements in exactly the manner that you see in Firebug, and browsers are pretty consistent in doing that.
But...
IE has at least one known bug in this area. Even IE9 exhibits this. Suppose the markup is this:
<!DOCTYPE html>
<title>Test case</title>
<form action='#'>
<input name="var1">
</form>
You should (and do in other browsers) get a DOM that looks like this:
HTML
HEAD
TITLE
BODY
FORM action="#"
INPUT name="var1"
But in IE you get this:
HTML
HEAD
TITLE
FORM action="#"
BODY
INPUT name="var1"
BODY
See it for yourself.
This bug seems limited to the form start tag preceding any text content and any body start tag.
The Google Style Guide for HTML recommends omitting all optional tags.
That includes <html> , <head> , <body> , <p> and <li> .
https://google.github.io/styleguide/htmlcssguide.html#Optional_Tags
For file size optimization and scannability purposes, consider omitting optional tags. The HTML5 specification defines what tags can be omitted.
(This approach may require a grace period to be established as a wider guideline as it's significantly different from what web developers are typically taught. For consistency and simplicity reasons it's best served omitting all optional tags, not just a selection.)
<!-- Not recommended -->
<!DOCTYPE html>
<html>
<head>
<title>Spending money, spending bytes</title>
</head>
<body>
<p>Sic.</p>
</body>
</html>
<!-- Recommended -->
<!DOCTYPE html>
<title>Saving money, saving bytes</title>
<p>Qed.
Contrary to @Liza Daly's note about HTML5, that spec is actually quite specific about which tags can be omitted, and when (and the rules are a bit different from HTML 4.01, mostly to clarify where ambiguous elements like comments and whitespace belong)
The relevant reference is http://www.w3.org/TR/2011/WD-html5-20110525/syntax.html#optional-tags, and it says:
An html element's start tag may be omitted if the first thing inside the html element is not a comment.
An html element's end tag may be omitted if the html element is not immediately followed by a comment.
A head element's start tag may be omitted if the element is empty, or if the first thing inside the head element is an element.
A head element's end tag may be omitted if the head element is not immediately followed by a space character or a comment.
A body element's start tag may be omitted if the element is empty, or if the first thing inside the body element is not a space character or a comment, except if the first thing inside the body element is a script or style element.
A body element's end tag may be omitted if the body element is not immediately followed by a comment.
So your example is valid HTML5, and would be parsed like this, with the html, head and body tags in their implied positions:
<!DOCTYPE html><HTML><HEAD>
<meta http-equiv="Content-type" content="text/html; charset=utf-8">
<title>Page Title</title>
<link rel="stylesheet" type="text/css" href="css/reset.css">
<script src="js/head_script.js"></script></HEAD><BODY><!-- this script will be in head //-->
<div>Some html</div> <!-- here body starts //-->
<script src="js/body_script.js"></script></BODY></HTML>
Note that the comment "this script will be in head" is actually parsed as part of the body, although the script itself is part of the head. According to the spec, if you want that to be different at all, then the </HEAD> and <BODY> tags may not be omitted. (Although the corresponding <HEAD> and </BODY> tags still can be)
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