Pixels can be expressed as decimals too.
But browsers render and round-up fractional ‘pixels’ differently, so reducing 1px to lesser values is likely to give inconsistent results across the board. A screen pixel isn’t the same as a print pixel either, so what about using
@media print to apply fractional border widths, but only for print. Working in point (pt) units might also prove more predictable in those circumstances too (points were made for print).
More info on print stylesheets https://www.sitepoint.com/create-a-customized-print-stylesheet-in-minutes/