[css-color] start conversion code for Rec.2020

Author: svgeesus

css-color/Overview.bs

@@ -1715,7 +1715,7 @@ These two colorspaces are indicated by using the predefined identifiers <i>P3</i
 <i>Rec.2020</i> in the color function. No ''<@profile>'' at-rule is needed; if one is 
 supplied, it will be ignored.
 
-The DCI P3[[P3]] colorspace has the following characteristics:
+The DCI P3[[!P3]] colorspace has the following characteristics:
 
 <table>
 	<tr><th>Red chromaticity</th><td>x=0.680 y=0.320</td></tr>
@@ -1725,7 +1725,7 @@ The DCI P3[[P3]] colorspace has the following characteristics:
 	<tr><th>Transfer function</th><td>1/2.6</td></tr>
 </table>
 
-The ITU Rec.2020[[2020]] colorspace has the following characteristics:
+The ITU Rec.2020[[!2020]] colorspace has the following characteristics:
 
 <table>
 	<tr><th>Red chromaticity</th><td>x=0.708 y=0.292</td></tr>
@@ -1735,15 +1735,13 @@ The ITU Rec.2020[[2020]] colorspace has the following characteristics:
 	<tr><th>Transfer function</th><td>see below</td></tr>
 </table>
 
-Rec.2020 defines two transfer functions, one for 10 bits per component 
-and one for 12 bits per component. Both use the same equation as sRGB, but with 
+Rec.2020 transfer function has the same form as the one for sRGB, but with 
 the constants at higher precision to cope with 10 or 12-bit components:
 
 <pre> E < β ? 4.5 * E : α * Math.pow(E, 0.45) - (α - 1);
 </pre>	
 
-For 10-bit, α=1.099 and β=0.018. For 12-bit, α=1.0993 and β=0.0181. 
-<!-- or give the full values, since this will be done in double precision float by JS anyway -->
+Where α=1.09929682680944  and β=0.018053968510807 . 
 
 For both predefined color spaces, conversion to Lab requires several steps, 
 although in practice all but the first step are linear calculations and can be combined.</p>
@@ -1777,18 +1775,20 @@ are the same for colors inside the source and destination gamuts.
 In a very similar way to the ''<@font-face.'' at-rule, the ''<@profile>'' at-rule has a descriptor 
 to give the profile a name which will be used inside the stylesheet, 
 <!-- is this actualy a descriptor, or a parameter to the at-rule?-->
-
-and 
-another descriptor to point to the actual data ('src', just like 'src'' in font-face.) 
+and another descriptor to point to the actual data ('src', just like 'src'' in font-face.) 
 <!-- not clear how I link to the two different descriptors therte, in bikeshed syntax -->
 
 <!-- should we add a format field to this descriptor, like with font-face src? it is always ICC format
 in practice, but that could change in the future. or we could add format later, once there is
-a need, ans make the default ICC if not specified -->
+a need, and make the default ICC if not specified -->
 
 In adition there is a third, optional descriptor to define the ''rendering intent'' to use. 
 This is only needed if the profile linked to has data for multiple rendering intents.
 
+<pre class="prod">
+	<dfn at-rule 
+</pre>
+
 <h2 id='cmyk-colors'>
 Device-dependent CMYK Colors: the ''device-cmyk()'' function</h2>