I made my portfolio default to dark on laptops and light on phones. Deciding the theme inside React guarantees a flash of the wrong one — the fix is a blocking inline script, and a MutationObserver to prove the flash is gone.
Contrast checkers test one colour pair. A gradient is an infinite number of pairs, and my indigo-to-cyan button passed at one end while failing badly at the other. The fix was a second gradient reserved for filled surfaces.
My Pages deploy failed twice with two different errors that both pointed nowhere. The real cause: the default Actions token is not allowed to create a Pages site, and the fix lives in a settings screen, not in the workflow.