The left column, vertical Home menu button of PixoBebo (http://pixobebo.com/) flickers in WebKit when the mouse hovers over Home (vertical menu only). The menu is CSS and properly validates. Also, the 'flicker' does not occur in the shipping version of Safari 3.1, or Firefox, or other browsers. Only in WebKit.
I cannot confirm the bug in Webkit r34053.
By "flickering", do you mean that the border of the button moves down by one pixel?
The style sheet specifies .navmenu li.active a { background: #3d3d52 url(http://pixobebo.com/images/images_pixobebo/navmenu_home_bg.png) no-repeat left center; [...] } and .navmenu li a:hover { background-position: -387px 0px; /* move 1st background image horizontally 387px, to start of 2nd background image */ [...] } Note the different vertical positions, 'center' vs. '0px'.
The same behavior can be seen in Firefox if the text size is increased or decreased.
The referenced page no longer has any problem, but it appears that the issue was simply that the author had some badly-written CSS. I'm closing this as invalid. If there is still an issue, please reopen the bug with a reduced testcase demonstrating it.