Android does allow you to enter fractional values for dp into XML, though I’m not sure I would recommend it because the results become less easy to predict/compute. Devices convert dp values into pixels using (basically) the following formula:
px = (int)(scale * dp + 0.5)
(i.e. the device density scale rounded to the nearest whole pixel value)
The scale value would be based on the screen density of the device:
- MDPI = 1
- HDPI = 1.5
- XHDPI = 2
- XXHDPI = 3
So 0.5dp would result in {1px, 1px, 1px, 2px} for the above densities, whereas 1dp would be {1px, 2px, 2px, 3px}. A tiny value like 0.1dp would resolve to {1px, 1px, 1px, 1px} because anything less than 1 in the above formula resolves to a single pixel unless the value was explicitly 0dp (it’s not possible to draw something thinner than a single pixel width).
If you want to ensure that thinnest possible width is used, probably best to define the width explicitly with px instead of a scaled unit like dp. Setting a value of 1px will draw with a single pixel on all devices, and is much more readable than hoping 0.5dp or 0.1dp does the same.