I am trying to do a liquid design.

So I started out to put all divs on top of each other before I would decide where to float what.

The weird thing. Between the first three divs there is a space of about 20px.

Even if I define margin to be 0px in the CSS, the gap doesn't go away.

Would you be able to post your CSS? If I could take a look at it I might be able to help you out.

There isn't a problem with my CSS. It passed the CSS validation without an error.

I think this is just what's called blockspace.

There's a design pattern that's called blockspace removal. Going to try this one out.

It's just weird that I have one example of divs joining without a blockspace.

You can pass the CSS validator with flying colors, and still have something that will give you unexpected results. If you can post your HTML and CSS, it would help us to help you find the problem. It's hard to just guess. It could be margin or padding on other elements, it could be the way you have thing laid out, or a number of other things that wouldn't cause a validation error, but could give you unexpected results.

It passed the CSS validation without an error.

Doesn't mean there isn't something wrong with the way it's coded

Sounds to me like margin-collapse at work, and that's not a bug. Try adding padding-top: 1px to one or more of the divs and see if the gap goes away. It happens when you stack divs like that.

Gotta see the code to know for certain.

It's gone.

First I made it go away by adding a negative margin.

Then I reworked the whole design and could finally throw away the negative margin.

I definitely messed something up at the beginning, but I can't tell any more what it was.