fix web-platform-tests#42496. better getCS with backgroundBlendMode

This was discussed in w3c/csswg-drafts#7164 (comment) > Don't specifically say the computed value is same as specified value, > it's implied. can be louder in the spec so it's obvious The specification for CSS values 4 makes it more explicit https://drafts.csswg.org/css-values-4/#linked-properties > The computed values of the coordinating list properties are > not affected by such truncation or repetition. So this patch add comments in the code to make sure this is understood and add a test to cover the case of multiple images.
karlcow · Oct 13, 2023 · 302bfb3 · 302bfb3
1 parent 80c0c84
commit 302bfb3
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diff --git a/css/compositing/parsing/background-blend-mode-computed-multiple.html b/css/compositing/parsing/background-blend-mode-computed-multiple.html
@@ -4,7 +4,7 @@
 <meta charset="utf-8">
 <title>Compositing and Blending Level 1: getComputedStyle().backgroundBlendMode</title>
 <link rel="help" href="https://drafts.fxtf.org/compositing-1/#propdef-background-blend-mode">
-<meta name="assert" content="background-blend-mode computed value is as specified.">
+<meta name="assert" content="background-blend-mode computed value is as specified even when the number of images vary.">
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <script src="/css/support/computed-testcommon.js"></script>
@@ -17,27 +17,33 @@
 <body>
 <div id="target"></div>
 <script>
+// This is testing the case with multiple background images
+//
 // see https://drafts.fxtf.org/compositing-1/#background-blend-mode
 // and https://drafts.csswg.org/css-backgrounds-3/#layering
 // > The lists are matched up from the first value: excess values at the end are not used.
 // and
 // > If a property doesn’t have enough comma-separated values
 // > to match the number of layers, the UA must calculate its used value
 // > by repeating the list of values until there are enough.
+// but in https://drafts.csswg.org/css-values-4/#linked-properties
+// it was decided that 
+// > The computed values of the coordinating list properties are not affected by such truncation or repetition.
+//
+// There is a distinction between specified values, used values, and computed values.
 
-// if three images and one value the initial value should be repeated.
-test_computed_value("background-blend-mode", "normal", "normal, normal, normal");
-test_computed_value("background-blend-mode", "multiply", "multiply, multiply, multiply");
+// if three images and one value, just send back the specified list.
+test_computed_value("background-blend-mode", "normal");
+test_computed_value("background-blend-mode", "multiply");
 
-// if three images and two values, just send back the same list and the first value for missing values.
-test_computed_value("background-blend-mode", "normal, luminosity", "normal, luminosity, normal");
-test_computed_value("background-blend-mode", "screen, overlay", "screen, overlay, screen");
-test_computed_value("background-blend-mode", "color, saturation", "color, saturation, color");
+// if three images and two values, just send back the specified list.
+test_computed_value("background-blend-mode", "normal, luminosity");
+test_computed_value("background-blend-mode", "screen, overlay");
+test_computed_value("background-blend-mode", "color, saturation");
 
-// if three images and three values, just send back the same list.
+// if three images and three values, just send back the specified list.
 test_computed_value("background-blend-mode", "normal, luminosity, color");
 test_computed_value("background-blend-mode", "screen, overlay, screen");
-
 </script>
 </body>
 </html>
diff --git a/css/compositing/parsing/background-blend-mode-computed.html b/css/compositing/parsing/background-blend-mode-computed.html
@@ -33,9 +33,14 @@
 // see https://drafts.fxtf.org/compositing-1/#background-blend-mode
 // and https://drafts.csswg.org/css-backgrounds-3/#layering
 // > The lists are matched up from the first value: excess values at the end are not used.
-test_computed_value("background-blend-mode", "normal, luminosity", "normal");
-test_computed_value("background-blend-mode", "screen, overlay", "screen");
-test_computed_value("background-blend-mode", "color, saturation", "color");
+// but in https://drafts.csswg.org/css-values-4/#linked-properties
+// it was decided that 
+// > The computed values of the coordinating list properties are not affected by such truncation or repetition.
+// There is a distinction between specified values, used values, and computed values.
+
+test_computed_value("background-blend-mode", "normal, luminosity");
+test_computed_value("background-blend-mode", "screen, overlay");
+test_computed_value("background-blend-mode", "color, saturation");
 </script>
 </body>
 </html>