[css-color-4] Define color value comparison algorithm #13157

Add a new "Comparing <<color>> Values" section immediately before
"Color Interpolation", defining when two <<color>> values are
"equivalent colors". The algorithm covers three cases: same color
space (component-by-component with epsilon, none only equals none),
different color spaces with a missing component (not equal), and
different color spaces with no missing components (convert to oklab,
compare with epsilon). Legacy sRGB syntaxes (rgb(), hsl(), hwb(),
hex, named, system colors) are normatively declared to share the
srgb color space. Used by style() container queries and CSS
Transitions to detect color changes.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: svgeesus

.claude/settings.local.json

@@ -0,0 +1,9 @@
+{
+  "permissions": {
+    "allow": [
+      "WebFetch(domain:github.com)",
+      "WebFetch(domain:api.github.com)",
+      "Bash(xargs grep:*)"
+    ]
+  }
+}

css-color-4/Overview.bs

@@ -4777,6 +4777,81 @@ where |src| and |dest| are <em>different</em>:
 </div>
 
 
+<!--
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+
+<h2 id="comparing-color-values">
+Comparing <<color>> Values</h2>
+
+	Two <<color>> values are <dfn export>equivalent colors</dfn>
+	when they compare as equal using the algorithm below.
+	This comparison is used,
+	for example,
+	by ''style()'' container queries [[CSS-CONDITIONAL-5]]
+	and by CSS Transitions [[CSS-TRANSITIONS-1]]
+	to determine whether a color value has changed.
+
+	Given two <<color>> values <var>C1</var> and <var>C2</var>,
+	they are [=equivalent colors=] if and only if
+	the following algorithm returns true:
+
+	<ol>
+		<li>
+			For each of <var>C1</var> and <var>C2</var>,
+			convert any [=powerless component=]s to [=missing component=]s.
+		</li>
+		<li>
+			If <var>C1</var> and <var>C2</var> share the same <<color-space>>:
+			<ol>
+				<li>
+					Compare their components one by one,
+					including the alpha channel.
+					A [=missing component=] is only equal to
+					another [=missing component=].
+					Two numeric components are considered equal
+					if they differ by no more than a small
+					implementation-defined ε.
+				</li>
+				<li>
+					Return true if and only if
+					all components compare as equal.
+				</li>
+			</ol>
+		</li>
+		<li>
+			Otherwise, <var>C1</var> and <var>C2</var> are in different <<color-space>>s.
+			If either color has at least one [=missing component=],
+			return false.
+		</li>
+		<li>
+			Otherwise, neither color has any [=missing component=].
+			Convert both <var>C1</var> and <var>C2</var> to ''oklab'',
+			then return true if and only if
+			all components (including alpha) of the converted colors
+			compare as equal,
+			using an implementation-defined ε as in step 2.
+		</li>
+	</ol>
+
+	Note: Two colors that are expressed in different <<color-space>>s
+	but are colorimetrically identical—for example,
+	''red'' and ''color(srgb 1 0 0)''—are [=equivalent colors=]
+	by step 4 of this algorithm,
+	since they convert to the same ''oklab'' value.
+
+	For the purposes of this comparison,
+	''rgb()'', ''rgba()'', ''hsl()'', ''hsla()'', ''hwb()'',
+	[=hex colors=], [=named colors=], and [=system colors=]
+	are all considered to be in the ''srgb'' <<color-space>>.
+
+
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@@ -7907,7 +7982,12 @@ Changes</h2>
 	<a href="https://www.w3.org/TR/2025/CRD-css-color-4-20250424/">Candidate Recommendation Draft of 24 April 2025</a></h3>
 
 <ul>
-	<!-- to 13 April 2026 -->
+	<!-- to 14 April 2026 -->
+	<li>Added a new section defining when two <<color>> values are [=equivalent colors=],
+		covering same-color-space component comparison, the treatment of [=missing component=]s,
+		and cross-color-space comparison via ''oklab''.
+		(<a href="https://github.com/w3c/csswg-drafts/issues/13157">Issue 13157</a>)
+	</li>
 	<li>Clarified in the main Color interpolation section that, if the hue interpolaton method is not specified, shorter is the default. (This was already specified in the Hue Interpolation section).
 	(<a href="https://github.com/w3c/csswg-drafts/issues/13788">Issue 13788</a>)
 	</li>