Set display none style on an element, and set the height of its child to something greater than 0, then the offsetHeight should return 0 for the parent element. HtmlUnit returns the height the child is set to and FF3 and IE8 both return 0. Attached html as example.
offsetHeight problem with alert
Fixed in SVN, thanks for the tiny test case! You can wait for 2.8 to be released, or you can download the snapshot JAR from http://build.canoo.com/htmlunit/artifacts/ after the next successful build.