PrincipleOfLeastMarking
The principle of least marking says that, if a UI element has multiple states, the default or inactive state should be represented by the visual variant that is least distinct from its surroundings.
To apply this principle, the elements to be distinguished must be embedded in a context that has some "dead space" not occupied by any element. One way UIs fail to apply this pattern is by putting a pair of elements in an environment with no disambiguating context.
On GitHub, inactive navigation tabs inherit the low-contrast bottom border of their context (the non-interactive portion of the navbar). The active tab has a markedly different border.
Another example from GitHub: the "write" and "preview" tabs of the wiki editor. Here the active tab is given a distinct background color as well as a marked border shape.
