How exactly are HSL colours meant to be written?

Question

I'm unclear about how HSL colours are meant to be written. W3 has them like this:

color: hsl(120, 100%, 50%);

Whereas ThreeJs asks for them like this:

h — hue value between 0.0 and 1.0 
s — saturation value between 0.0 and 1.0 
l — lightness value between 0.0 and 1.0

I've seen HSL colours written both ways across the web and really don't get it - which is correct, and how do you convert between the two?

Cheers.

Answer 1

For CSS, the W3C Recommendation CSS Color Module Level 3 defines how HSL color values have to be specified in the color property:

HSL colors are encoding as a triple (hue, saturation, lightness). Hue is represented as an angle of the color circle (i.e. the rainbow represented in a circle). This angle is so typically measured in degrees that the unit is implicit in CSS; syntactically, only a is given. […] Saturation and lightness are represented as percentages.

tl;dr:

1. hue: unitless
2. saturation: percentage
3. lightness: percentage

The next version, CSS Color Module Level 4 (which is currently only an Editor’s Draft) specifies this better (and it seems that the hue argument can have more values):

HSL colors are specified as a triplet of hue, saturation, and lightness. The syntax of the hsl() function is:

hsl() = hsl( <hue>, <percentage>, <percentage> )  
hsla() = hsla( <hue>, <percentage>, <percentage>, <alpha-value> )
<hue> = <number> | <angle> | <named-hue>

Answer 2

For CSS hue value must be between 0 and 360 degrees, saturation and lightness must be between 0 and 100 percents, alpha channel must be between 0 and 1. You may use Regular Expressions to parse string CSS color and get the numeric values:

function parseHsl(color) {
    var hslRegexp = /^\s*hsl\s*\(\s*(\d{1,3}\.?\d?)\s*,\s*(\d{1,3}\.?\d*)%\s*,\s*(\d{1,3}\.?\d*)%\s*\)\s*$/
    var match = color.match(hslRegexp);
    if (!match) {
        throw new Error('Not an HSL color.');
    }
    var h = +match[1];
    var s = +match[2];
    var l = +match[3];
    if (isNaN(h) || isNaN(s) || isNaN(l)) {
        throw new Error('Not a number.');
    }
    h = h / 360;
    s = s / 100;
    l = l / 100;
    if (h < 0 || h > 1 || s < 0 || s > 1 || l < 0 || l > 1) {
        throw new Error('Out of range.');
    }
    return {
        h: h,
        s: s,
        l: l
    };
}

function getCssHslColor(hsl) {
    return 'hsl(' + hsl.h * 360 + ',' + hsl.s * 100 + '%,' + hsl.l * 100 + '%)';
}