The Key — The Columns Are Not Really Equal
The only way to do this without using a very complex CSS layout is to essentially create the illusion that you have equal columns. But don’t worry — although the result is not really equal columns, it looks and acts exactly as an equal column layout would.
How to Mimic the Columns
To describe the method, I’ll just use a simple example: Two DIVs side by side representing the two columns; and one DIV containing the two. From there, it’s quite simple — just wrap your containing DIV with another DIV then apply “overflow: hidden” on that outer DIV to ensure it doesn’t collapse in Firefox. Then, add a background image that repeats on the Y-axis (vertically). So, if you have a drop shadowed border, or even a simple 1 pixel border, you would create the image in your image editing program (Photoshop, Illustrator, etc.) and then just adjust the background-position of the image in your CSS to place it exactly where you want it. Since this DIV is essentially behind your two columns, holding both of them, you cannot have a background color on your columns, as it would block the image from showing up.
Keep in Mind
You might think this could be done with a one pixel-border on one of the columns, but remember, one of the columns is not going to extend to the bottom, so the border method won’t work without some serious CSS hacks and workarounds. You’re creating the illusion that the short column is extending to the bottom by positioning the background image in a place where it mimics the short column’s border. Except, unlike the short column, your outer container (the container holding the container, if you will) is always going to extend to the bottom, thus creating the illusion of a two column layout where the columns are always equal, regardless of content. And best of all, either of the two columns could extend larger than the other, and the illusion would still work.
Overall, a Very Easy Solution With Limited Drawbacks
This really is an easy to implement solution that can work on many CSS layouts, and doesn’t have too many drawbacks, other than adding some non-semantic code to your markup and (somewhat) complicating your DIV structure.
So if you’re in need of a hack-free quick fix to a two-column layout, this is a good solution and will work in virtually every browser and platform.
View the demo page, to see this simple, yet effective, solution in action.