[css-color-5] consistent heading syntax

Author: svgeesus

css-color-4/Overview.bs

@@ -4353,8 +4353,8 @@ Simple alpha compositing</h3>
 	When drawing, implementations must handle alpha
 	according to the rules in <a href="https://www.w3.org/TR/compositing-1/#simplealphacompositing">Section 5.1 Simple alpha compositing</a> of [[!Compositing]].
 
-	Interpolation {#interpolation}
-	===================================
+<h2 id="interpolation">
+Interpolation</h2>
 
 	Color interpolation happens with
 	gradients,
@@ -4381,8 +4381,8 @@ Simple alpha compositing</h3>
 	em { color: red; }</code>
 	See <a href="https://github.com/w3c/csswg-drafts/issues/445">Issue 445</a>.
 
-	Color space for interpolation {#interpolation-space}
-	------------------------------
+<h3 id="interpolation-space">
+Color space for interpolation</h3>
 
 	Issue: Should gamut mapping occur before or after interpolation?
 
@@ -4392,9 +4392,8 @@ Simple alpha compositing</h3>
 	Host syntax can override the interpolation color space and specify which color space is used for interpolation.
 	For example, 'color-mix' and 'color-adjust' override the default to LCH.
 
-
-	Interpolating with alpha {#interpolation-alpha}
-	------------------------
+<h3 id="interpolation-alpha">
+Interpolating with alpha</h3>
 
 	When the colors to be interpolated are not fully opaque,
 	they are transformed into <dfn export>premultiplied color values</dfn>
@@ -4450,8 +4449,8 @@ Simple alpha compositing</h3>
 
 	Issue(445):
 
-	Hue interpolation {#hue-interpolation}
-	-------------------
+<h3 id="hue-interpolation">
+Hue interpolation</h3>
 
 	Issue(4928): How to handle achromatic interpolation?