[css-color-5] Preserve hsl/hwb identity when result contains none, #10254

Per CSSWG resolution 2026-01-27, defer to the css-color-4
sRGB serialization rules: an hsl() or hwb() color-mix or
relative-color result that contains a missing component
(including an alpha carried forward from the origin color)
serializes using the modern hsl(H S L / A) or hwb(H W B / A)
form, rather than degrading to color(srgb r g b) which would
lose the original color-function identity.

Adds a worked example for the original test case
"hsl(from rebeccapurple none none none / none)".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: svgeesus

css-color-5/Overview.bs

@@ -3325,6 +3325,16 @@ depends on the color space specified with "in":
 
 </dl>
 
+However, if the result of mixing in the ''hsl'' or ''hwb'' [=color space=]
+has at least one [=missing color component=]
+(including a [=missing=] alpha [=carried forward=]
+per [[css-color-4#interpolation-missing]]),
+the form used is the modern ''hsl(h s l / a)'' or ''hwb(h w b / a)'' syntax respectively,
+preserving the original color function and each ''none'' value
+per [[css-color-4#css-serialization-of-srgb]],
+rather than degrading to ''color(srgb r g b)'' (which would lose
+the ''hsl''/''hwb'' identity).
+
 <wpt>
 	parsing/color-valid-color-mix-function.html
 	color-mix-currentcolor-visited-getcomputedstyle.html
@@ -3508,6 +3518,23 @@ while the serialization of the computed value
 is the string "color(srgb 0.55 0.45 0.45)".
 </div>
 
+<div class="example" id="ex-serial-rcs-alpha-none">
+For example, the serialization of the declared value of
+
+<pre class="lang-css">hsl(from rebeccapurple none none none / none);</pre>
+
+is the string "hsl(from rebeccapurple none none none / none)".
+
+The computed value carries forward the [=missing=] alpha
+(alpha is its own [=analogous components|analogous component=]),
+giving an ''hsl()'' [=relative color=]
+whose hue, saturation, lightness, and alpha are all [=missing=].
+Because the resolved value contains [=missing color components=],
+the serialization uses the modern ''hsl()'' form,
+yielding the string "hsl(none none none / none)"
+rather than ''color(srgb 0 0 0 / none)''.
+</div>
+
 <div class="example" id="ex-serial-rcs-hsl-unclamped">
 For example, the serialization of the declared value of
 
@@ -3616,6 +3643,19 @@ depends on the color space of the [=relative color=]:
 	</tr>
 </table>
 
+However, if the resolved value of an ''hsl()'' or ''hwb()'' [=relative color=]
+has at least one [=missing color component=]
+(including a [=missing=] alpha [=carried forward=]
+from the [=origin color=] per [[css-color-4#interpolation-missing]]),
+the form used is the modern ''hsl(h s l / a)'' or ''hwb(h w b / a)'' syntax respectively,
+preserving the original color function and each ''none'' value,
+rather than degrading to ''color(srgb r g b)'' (which would lose
+the ''hsl''/''hwb'' identity).
+This parallels [[#serial-origin-color]],
+which always emits the modern slash syntax for origin colors,
+and follows the general sRGB serialization rules in
+[[css-color-4#css-serialization-of-srgb]].
+
 <wpt>
 
 </wpt>