This solution is generated by ChatGPT, correctness is not guaranteed.

AI solution for CSS coding question on BFE.dev
14. 0.5px border

.hairline { width: 150px; border-top: 0.5px solid black; }

Note: Please note that 0.5px borders are not always visible on all devices and may not be supported by some browsers. It's always a safe practice to use a minimum of 1px border size for consistency and accessibility.