On my PC, the line is just a single pixel. On my Nexus 5 -- as you can see -- it's actually three lines, which are (from top to bottom): grey (RGB=134), black (RGB=0), grey (RGB=156).
The impression I get, when looking at it on my phone, is a solid black line. Bear in mind, that the width of the display is 61 millimeters, which means that these three pixels take up only 0.17 millimeters on the screen.
Here's the web page with the 1 pixel line: http://jsfiddle.net/k0xep8pL/embedded/result/
On my PC, the line is just a single pixel. On my Nexus 5 -- as you can see -- it's actually three lines, which are (from top to bottom): grey (RGB=134), black (RGB=0), grey (RGB=156).
The impression I get, when looking at it on my phone, is a solid black line. Bear in mind, that the width of the display is 61 millimeters, which means that these three pixels take up only 0.17 millimeters on the screen.