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I have several Wordpress blogs and regular HTML websites help would be appreciated for both types of sites.
There are a variety of ways in which a fixed width element can be included in a web page. Without taking your coding apart it would be pure guesswork and could be anything from absolute positioning in a stylesheet to the size and position of an image.
People often have this problem when they use third party templates for their site and don't know exactly what it is doing.
Unfortunately, there is no easy way to do this. You can use percentages to help control the width, but I understand that if you are trying to control a few things - using a fluid layout can be difficult.
The problem lies is assumptions. First, you are assuming the user has his browser maximized. This might be the case on some resolutions, but possibly not on higher resolutions. For example, I have 1920X1200 resolution, but Microsoft IE is set up at 1280X1024. To my knowledge, you can detect resolution but you cannot detect what size I might have.
Plus, you are assuming the user does not have the favorites / history tool bar opened. This could cost you about 300 pixels or more.