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Sidebars have fixed width and main column takes up the rest of the available space.

How-to

Set float on all columns. Set main column width to 100%, and sidebars to some known width. This could be pixel or percentage value.

Use negative margin to create extra space where sidebars should fit. Now, some columns will overlap. Add margin to content inside main column to prevent this overlap.

Content on Left: Sidebars have to fit on the right. Float main column to left and set its right-margin to negative value.

Content on Right: Sidebars have to fit on the left. Float main column to right and set its left-margin to negative value.

Content in Center: Float main column to right and set its left margin to -100%. Float sidebars as needed.