User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.12) Gecko/20080207 Ubuntu/7.10 (gutsy) Firefox/2.0.0.12 Build Identifier: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.12) Gecko/20080207 Ubuntu/7.10 (gutsy) Firefox/2.0.0.12 When you hold down the left mouse button an html form button (e.g. <input type="submit"/>), the button takes on the css :active pseudo-class, which makes it look pressed in by default. Then when you let go of the mouse button it pops back up and the form submits. But if you navigate to the button with the tab key, then hold down the space bar, there is no indication that the button action is about to happen when you release the space bar. Regular gtk+ buttons are depressed when you press space. I'm pretty sure it's generally true in all guis. And the space bar situation is totally analogous to the mouse click situation. Reproducible: Always