For a readable interface, it's important to use visual cues to group or separate content to show which content relates to other content. This makes content easier to read, especially for people who have difficulty reading on-screen content and people who use screen readers.
When grouping content visually, you must make sure that you're not skewing the source order and breaking up the same content for screen-reader users.
When positioning elements using CSS, it's possible to place them in a position that does not reflect their location in the source order. This can have unexpected side effects for keyboard-navigation users.