I am currently learning html/css, and have noticed a common technique is to place a generic container div in the root of the body tag:
<html>
<head>
...
</head>
<body>
<div id="container">
...
</div>
</body>
</html>
Is there a valid reason for doing this? Why can't the css just reference the body tag?
THis method allows you to have more flexibility of styling your entire content. Effectivly creating two containers that you can style. THe HTML Body tag which serves as your background, and the div with an id of container which contains your content.
This then allows you to position your content within the page, while styling a background or other effects without issue. THink of it as a "Frame" for the content.
The container div, and sometimes content div, are almost always used to allow for more sophisticated CSS styling. The body tag is special in some ways. Browsers don't treat it like a normal div; its position and dimensions are tied to the browser window.
But a container div is just a div and you can style it with margins and borders. You can give it a fixed width, and you can center it with margin-left: auto; margin-right: auto.
Plus, content, like a copyright notice for example, can go on the outside of the container div, but it can't go on the outside of the body, allowing for content on the outside of a border.
The most common reasons for me are so that:
This is one of the biggest bad habits perpetrated by front end coders.
All the answers above me are wrong. The body does take a width, margins, borders, etc and should act as your initial container. The html element should act as your background "canvas" as it was intended. In dozens of sites I've done I've only had to use a container div once.
I'd be willing to be that these same coders using container divs are also littering their markup with divs inside of divs--everywhere else.
Dont' do it. Use divs sparingly and aim for lean markup.
Certain browsers (<cough> Internet Explorer) don't support certain properties on the body, notably width and max-width.