function rgb2hsl(rgb) {
  var r = rgb[0]/255,
      g = rgb[1]/255,
      b = rgb[2]/255,
      min = Math.min(r, g, b),
      max = Math.max(r, g, b),
      delta = max - min,
      h, s, l;

  if (max == min)
    h = 0;
  else if (r == max)
    h = (g - b) / delta;
  else if (g == max)
    h = 2 + (b - r) / delta;
  else if (b == max)
    h = 4 + (r - g)/ delta;

  h = Math.min(h * 60, 360);

  if (h < 0)
    h += 360;

  l = (min + max) / 2;

  if (max == min)
    s = 0;
  else if (l <= 0.5)
    s = delta / (max + min);
  else
    s = delta / (2 - max - min);

  return [h, s * 100, l * 100];
}
Ported from sass implementation in C
https://github.com/sass/libsass/blob/0e6b4a2850092356aa3ece07c6b249f0221caced/functions.cpp#L209