RenderProgress should not need to inherit from RenderBlock Same logic as bug 84421. It has no visible (text) children, and really is just a repalced inline element. Should likely inherit from RenderReplaced. progress { -webkit-appearance: progress-bar; -webkit-box-sizing: border-box; display: inline-block; height: 1em; width: 10em; vertical-align: -0.2em; } At least that's my understanding. It does not appear to have rendered children?
After discussion with Hyatt, this needs to be a RenderBlock in order to contain the renderers from its shadow children.