How to draw a vector line in millimeters?

Question

SVG

I draw a red line 10 pixels long like this:

<line x1='0' y1='0' y2='0' x2='10' style='stroke:rgb(255,0,0); stroke-width:1' />

But how to draw a line 10 mm long?
If we take the known ratio: 1 pixel \u003d 0.264583333333334 millimeter, we get a HUGE UNACCEPTABLE ERROR

Answer 1

well, indicate that these are millimeters:
10mm