Get equal heights for 3 column web page

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I am making a 3 column web page - 2 sidebars on either side (having fixed content) and a main content block in the centre (having dynamic content). It'll be like a blog; when a user is first using it, the main content block will contain very little stuff. So its height will be less than those of the sidebars. But as the block fills up, its height will become more than the sidebars'. I want all the 3 columns to have the same height - equal as that of the one having the longest height. How do I achieve this? P.S. I would like to use JavaScript only if it is *absolutely* necessary. Regards.