BFE.devAI解答 - CSS 问题
14. 0.5px border

Unfortunately, it's not possible to add a 0.5px border using pure CSS because browsers round up to the nearest whole pixel value. One workaround is to use a technique called sub-pixel rendering using CSS transforms. Here's how you can add a 0.5px border to the top of the given div using sub-pixel rendering:

.hairline {
  width: 150px;
  border-top: 1px solid black;
  transform: scaleY(0.5);
}

This code first sets a 1px top solid border in black color. Then, it uses the transform property to scale the div along the Y axis by a factor of 0.5, effectively making the border half the width of a regular 1px border.

Note that this technique is not recommended for all scenarios because it can cause some blurriness or distortion on certain displays, especially if the div is animated or changes size frequently.