As others have already mentioned, HTML 4 requires the <style> tag to be placed in the <head> section (even though most browsers allow <style> tags within the body).
However, HTML 5 includes the scoped attribute (see update below), which allows you to create style sheets that are scoped within the parent element of the <style> tag. This also enables you to place <style> tags within the <body> element:
<!DOCTYPE html>
<html>
<head></head>
<body>
<div id="scoped-content">
<style type="text/css" scoped>
h1 { color: red; }
</style>
<h1>Hello</h1>
</div>
<h1>
World
</h1>
</body>
</html>
If you render the above code in an HTML-5 enabled browser that supports scoped, you will see the limited scope of the style sheet.
There’s just one major caveat…
At the time I’m writing this answer (May, 2013) almost no mainstream browser currently supports the scoped attribute. (Although apparently developer builds of Chromium support it.)
HOWEVER, there is an interesting implication of the scoped attribute that pertains to this question. It means that future browsers are mandated via the standard to allow <style> elements within the <body> (as long as the <style> elements are scoped.)
So, given that:
- Almost every existing browser currently ignores the
scopedattribute - Almost every existing browser currently allows
<style>tags within the<body> - Future implementations will be required to allow (scoped)
<style>tags within the<body>
…then there is literally no harm * in placing <style> tags within the body, as long as you future proof them with a scoped attribute. The only problem is that current browsers won’t actually limit the scope of the stylesheet – they’ll apply it to the whole document. But the point is that, for all practical purposes, you can include <style> tags within the <body> provided that you:
- Future-proof your HTML by including the
scopedattribute - Understand that as of now, the stylesheet within the
<body>will not actually be scoped (because no mainstream browser support exists yet)
* except of course, for pissing off HTML validators…
Finally, regarding the common (but subjective) claim that embedding CSS within HTML is poor practice, it should be noted that the whole point of the scoped attribute is to accommodate typical modern development frameworks that allow developers to import chunks of HTML as modules or syndicated content. It is very convenient to have embedded CSS that only applies to a particular chunk of HTML, in order to develop encapsulated, modular components with specific stylings.
Update as of Feb 2019, according to the Mozilla documentation, the scoped attribute is deprecated. Chrome stopped supporting it in version 36 (2014) and Firefox in version 62 (2018). In both cases, the feature had to be explicitly enabled by the user in the browsers’ settings. No other major browser ever supported it.