For Mac OS users, there is a system setting that may not allow you to tab onto several types of elements in a web page. To change this setting:
Like a Windows laptop your Chromebook has a ‘Taskbar’ known as the ‘Shelf.’
From here you can access settings, wifi network(s), sound, brightness, ect.
As your device runs web apps you may also find ‘Kiosk’ or ‘Launch’ apps in the Shelf.
The ‘Apps’ seen below is used primarily for testing and doesn’t require log-in information since it takes users into secure browsers.