When focus order is broken, commonly by CSS positioning that visually rearranges elements without a matching change in the underlying HTML order, keyboard users can end up jumping erratically around the page, skipping content, or landing on elements that make no sense in isolation.
WCAG success criterion 2.4.3 requires a meaningful focus order, and it’s one of the fastest things to test manually: unplug the mouse, press Tab repeatedly, and see whether the highlighted element’s order makes sense from top to bottom, left to right.