[css-color-5] Hue interpolation fixes, per @dbaron’s suggestion

Author: LeaVerou

css-color-5/Overview.bs

@@ -727,10 +727,14 @@ Hue interpolation {#hue-interpolation}
 -------------------
 
 For color functions with a hue angle (LCH, HSL, HWB etc), there are multiple ways to interpolate.
-We typically want to avoid arcs over 360, as they are rarely desirable,
+We typically want to avoid arcs over 360 for the difference between the angles, as they are rarely desirable,
 so in most cases angles are fixed up prior to interpolation so that per-component interpolation is done over less than 360 degrees, often less than 180.
+
 Host syntax can specify any of the following algorithms for hue interpolation
-(angles in the following are in degrees, but the logic is the same regardless of how they are specified):
+(angles in the following are in degrees, but the logic is the same regardless of how they are specified).
+
+Unless the type of hue interpolation is ''specified'', both angles need to be constrained to [0, 360) prior to interpolation.
+One way to do this is <code><i>θ</i> = ((<i>θ</i> % 360) + 360) % 360</code>.
 
 : ''shorter''
 :: Angles are adjusted so that θ₂ - θ₁ ∈ [0, 180). In pseudo-Javascript:
@@ -746,11 +750,11 @@ Host syntax can specify any of the following algorithms for hue interpolation
 : ''longer''
 :: Angles are adjusted so that θ₂ - θ₁ ∈ [180, 360). In pseudo-Javascript:
 	<pre>
-	if (θ₂ - θ₁ <= 180) {
-		θ₂ += 360;
+	if (0 < θ₂ - θ₁ < 180) {
+	  θ₁ += 360;
 	}
-	else if (θ₂ - θ₁ >= -180) {
-		θ₁ += 360;
+	else if (-180 < θ₂ - θ₁ < 0) {
+	  θ₂ += 360;
 	}
 	</pre>
 
@@ -775,7 +779,6 @@ Host syntax can specify any of the following algorithms for hue interpolation
 
 Unless otherwise specified, if no specific hue interpolation algorithm is selected by the host syntax, the default is ''shorter''.
 
-
 Security and Privacy Considerations {#SecPriv}
 ===================================