We’re trying to transition the `opacity` property only. When at `opacity:0`, the content can’t be seen but we can see the space it occupies in the page, and the links and other focusable elements are still part of the tab order.
(Visible/invisible states are triggered by changing a class with JavaScript. Transition time is sluggish — 2 seconds — to make the browsers’s behaviors more obvious.)
Test link after the hidden content.