Well I would suggest implementing at least at the level of modern desktops, and make it easy to find and use it, not just alt+tab shortcut. Ideally it should be spawned and allow the user to filter to find tabs instead of having to hold and release a key. My point being, a simple forward/backward cycle is definitely on the lower side of effort when it comes to window management (tab management), which to be honest is not a problem to me personally, but I find the gap left there on purpose bizarre and makes me question it. I don't think browsers should consider themselves just rendering frames since it is the interface that users use, it's not a component, and it should be (very) ambitious when it comes to advanced features, especially with something as fundamental to them as tab switching.