[css-color-4] define host syntax, see w3c#6439

Author: svgeesus

css-color-4/Overview.bs

@@ -4442,7 +4442,22 @@ Color space for interpolation</h3>
 	while the linear-light XYZ is best for compositing
 	and gamma-encoded sRGB, which is neither linear light nor perceptually uniform,
 	will give overly dark mixes but is compatible with older Web content.
-	The host syntax may define what the default interpolation space should be for each case,
+
+	Various features in CSS depend on interpolating colors.
+
+	<div class="example" id="ex-host-syntax">
+	Examples include:
+
+		* <<gradient>>
+		* 'filter'
+		* 'animation'
+		* 'transition'
+		* [[color-mix()]]
+	</div>
+
+	These features are collectively termed the<dfn>host syntax</dfn>.
+	The host syntax should define
+	what the default interpolation space should be for each case,
 	and optionally provide syntax for authors to override this default.
 
 	If the host syntax does not define what color space
@@ -4941,6 +4956,7 @@ Changes</h2>
 </h3>
 
 <ul>
+	<li>Defined the term host syntax</li>
 	<!-- to 9 July 2021 -->
 	<li>Added Lea Verou as co-editor by WG resolution</li>
 	<li>Clarified that hue angle is unbounded</li>