HSL and HSLA

Hello,

Could you explain why the hierarchy on the CSS-3 color page 
<http://www.w3.org/TR/css3-color/> is

4.2.1. RGB color values
4.2.2. RGBA color values
4.2.3. 'transparent' color keyword
4.2.4. HSL color values
	4.2.4.1. HSL Examples
4.2.5. HSLA color values

and not

4.2.1. RGB color values
	4.2.1.1. RGBA color values
4.2.2. 'transparent' color keyword
4.2.3. HSL color values
	4.2.3.1. HSL Examples
	4.2.3.2. HSLA color values

I don't understand the point of separating RGA and RGBA and HSL and 
HSLA. Can you explain?

Thank you

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