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Editorial Team
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Asked: 2026-05-13T22:55:50+00:00

Most of javascript and web development books/articles says that you must put CSS in

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Most of javascript and web development books/articles says that you must put CSS in the head tag and javascript at the bottom of the page.

But when I open html source of famous websites such as this one stackoverflow, I find they put some js files in the head tag.

What’s Pros and Cons of both approaches and when to use which?

Found another question for the same issue:
Where should I declare JavaScript files used in my page? In <head></head> or near </body>?

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1 Answer

  1. Editorial Team

    From Yahoo’s Best Practices for Speeding Up Your Web Site:

    The problem caused by scripts is that
    they block parallel downloads. The
    HTTP/1.1 specification suggests that
    browsers download no more than two
    components in parallel per hostname.
    If you serve your images from multiple
    hostnames, you can get more than two
    downloads to occur in parallel. While
    a script is downloading, however, the
    browser won’t start any other
    downloads, even on different
    hostnames.

    In some situations it’s not easy to
    move scripts to the bottom. If, for
    example, the script uses
    document.write to insert part of the
    page’s content, it can’t be moved
    lower in the page. There might also be
    scoping issues. In many cases, there
    are ways to workaround these
    situations.

    An alternative suggestion that often
    comes up is to use deferred scripts.
    The DEFER attribute indicates that the
    script does not contain
    document.write, and is a clue to
    browsers that they can continue
    rendering. Unfortunately, Firefox
    doesn’t support the DEFER attribute.
    In Internet Explorer, the script may
    be deferred, but not as much as
    desired. If a script can be deferred,
    it can also be moved to the bottom of
    the page. That will make your web
    pages load faster.

    Therefore, in general, it is preferrable to put them at the bottom. However, it isn’t always possible, and it often doesn’t make that much of a difference anyway.

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