[css-color] explicit sections for predefined conversions

Author: svgeesus

css-color/Overview.bs

@@ -1744,6 +1744,8 @@ the constants at higher precision to cope with 10 or 12-bit components:
 
 Where α=1.09929682680944  and β=0.018053968510807 . 
 
+<h4 id="predefined-to-lab">Converting predefined colorspaces to Lab</h4>
+
 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>
 
@@ -1755,6 +1757,8 @@ although in practice all but the first step are linear calculations and can be c
 	<li>Convert D50-adapted XYZ to Lab
 </ol>
 
+<h4 id="lab-to-predefined">Converting Lab to predefined colorspaces</h4>
+
 Conversion from Lab to P3 or Rec.2020 also requires multiple steps, and again in practice all 
 but the last step are linear calculations and can be combined.