Relative luminance of the color

// As heard on "Chroma Zone" talk by Lea Verou
// Explanation @ http://www.w3.org/TR/2014/NOTE-WCAG20-TECHS-20140311/G18
// @color - [r,g,b]

function luminance (color) {
    var rgb = color.map(function(c){
		c /= 255; // to 0-1 range
		return c < .03928 ? 
		         c / 12.92 : 
		         Math.pow((c+.055)/1.055, 2.4);
	});
	
	return 21.26 * rgb[0] + // red
	       71.52 * rgb[1] + // green
	        7.22 * rgb[2];  // blue
}

/* 
1. calculate luminance for text and background
2. Calculate the contrast ratio using the following formula. 
 (L1 + 0.05) / (L2 + 0.05), where L1 is the relative luminance of the lighter of the foreground or background colors, and L2 is the relative luminance of the darker of the foreground or background colors.
3. Check that the contrast ratio is equal to or greater than 4.5:1
*/