When you add a doctype the browser works in standards mode and most of the time won’t apply dodgy legacy behaviour . 100% height specifically is not as simple as you may think.
To be 100% high an element has to be 100% of something else. As you haven’t defined any height for any parent then the height collapses to zero. Without a doctype the browser will guess what you want which is why beginners often prefer quirks mode because it lets them get away with bad coding.
While thats good for beginners its bad for structured code
Have a look a the faq on 100% height for a clear explanation.
If you are looking for equalising columns then you should look at the 3 col demos as wel for some hints and tips.
I think I know, it couldn’t collaps, because the two neighboring divs were supporting it’s box, since I had it between them, and display set to inline.
If the elements you are reffering to are floats as in your previous post then setting them to display:inline means nothing (other than it cures ie’s double margin bug). Floats are hard coded as display block and cannot be changed.
I think you need to read my faq on 100% height as you are still not quite sure of the workings :). Even if you accomplish 100% height for your columns in the manner you are trying you will not get anything more than viewport height (in good browsers) and the columns will not automatically grow together.
If this didn’t affect MSIE6, then I think you have entered a doctype that doesn’t switch MSIE into standards mode. You should have an URL in the doctype definition
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"