Unable to log in to Steam using a web browser (Chrome, Opera, FireFox, IE)

I am able to log in to my Steam account using the Steam application, but when using the same username and password through a web browser, the Steam website indicates that my credentials are incorrect. What can I do?

Removing Extensions that may interfere with the login process

Try disabling any browser extensions or add-ons you have installed. These may interfere with the login process.

For more information on how to remove extensions or add-ons with common browsers please visit the following links:

Google Chrome: http://support.google.com/chrome/bin/answer.py?hl=en&answer=113907

Mozilla Firefox: http://support.mozilla.org/en-US/kb/disable-or-remove-add-ons

Attempt to log in with a different, unmodified web browser

If you are unable to log into your Steam account with your current web browser, please try using a different browser and test the issue.

You may wish to perform a clean install of that web browser, in case any older extensions or addons may still be interfering with this process.