Bug 1836848 - Avoid selecting zero-resolution images in image-set. r=dholbert

Fairly straight-forward. This fixes the two tests mentioned in comment 0
which aren't still in the repo.

Differential Revision: https://phabricator.services.mozilla.com/D180414

servo/components/style/values/computed/image.rs

@@ -80,24 +80,28 @@ impl ToComputedValue for specified::ImageSet {
 
         let mut supported_image = false;
         let mut selected_index = std::usize::MAX;
-        let mut selected_resolution = items[0].resolution.dppx();
+        let mut selected_resolution = 0.0;
 
         for (i, item) in items.iter().enumerate() {
-            // If the MIME type is not supported, we discard the ImageSetItem
             if item.has_mime_type && !context.device().is_supported_mime_type(&item.mime_type) {
+                // If the MIME type is not supported, we discard the ImageSetItem.
                 continue;
             }
 
             let candidate_resolution = item.resolution.dppx();
+            debug_assert!(candidate_resolution >= 0.0, "Resolutions should be non-negative");
+            if candidate_resolution == 0.0 {
+                // If the resolution is 0, we also treat it as an invalid image.
+                continue;
+            }
 
             // https://drafts.csswg.org/css-images-4/#image-set-notation:
             //
-            //     Make a UA-specific choice of which to load, based on whatever
-            //     criteria deemed relevant (such as the resolution of the
-            //     display, connection speed, etc).
+            //     Make a UA-specific choice of which to load, based on whatever criteria deemed
+            //     relevant (such as the resolution of the display, connection speed, etc).
             //
-            // For now, select the lowest resolution greater than display
-            // density, otherwise the greatest resolution available
+            // For now, select the lowest resolution greater than display density, otherwise the
+            // greatest resolution available.
             let better_candidate = || {
                 if selected_resolution < dpr && candidate_resolution > selected_resolution {
                     return true;

testing/web-platform/meta/css/css-images/image-set/image-set-zero-resolution-rendering-2.html.ini

@@ -1,2 +0,0 @@
-[image-set-zero-resolution-rendering-2.html]
-  expected: FAIL

testing/web-platform/meta/css/css-images/image-set/image-set-zero-resolution-rendering.html.ini

@@ -1,2 +0,0 @@
-[image-set-zero-resolution-rendering.html]
-  expected: FAIL