As far as I know you CANNOT use a half pixel UNLESS you are styling a text element...
GOOD EXAMPLE:
#myDiv {
font-size: 12.5px;
}
BAD EXAMPLE:
#myDiv {
width: 350.5px;
}
I think the common misunderstanding is that a pixel is some relative form of measurement. A pixel is a standardized unit and the smallest usable screen element. A pixel contains only one solid color created by mixing light intensities such as red, green, and blue(RGB), or cyan, magenta, yellow, and black(CMYK).
I hope this helps a little bit. You can definitely find more in depth info if you keep searching.