diff --git a/components/style/values/computed/image.rs b/components/style/values/computed/image.rs
index fc484979153..8844f7306c0 100644
--- a/components/style/values/computed/image.rs
+++ b/components/style/values/computed/image.rs
@@ -78,10 +78,8 @@ impl ToComputedValue for specified::ImageSet {
         let items = self.items.to_computed_value(context);
         let dpr = context.device().device_pixel_ratio().get();
 
-        // If no item have a supported MIME type, the behavior is undefined by the standard
-        // By default, we select the first item
         let mut supported_image = false;
-        let mut selected_index = 0;
+        let mut selected_index = std::usize::MAX;
         let mut selected_resolution = items[0].resolution.dppx();
 
         for (i, item) in items.iter().enumerate() {
@@ -126,7 +124,7 @@ impl ToComputedValue for specified::ImageSet {
 
     fn from_computed_value(computed: &Self::ComputedValue) -> Self {
         Self {
-            selected_index: 0,
+            selected_index: std::usize::MAX,
             items: ToComputedValue::from_computed_value(&computed.items),
         }
     }
diff --git a/components/style/values/generics/image.rs b/components/style/values/generics/image.rs
index 3b84d982381..7e0ac7ec5b2 100644
--- a/components/style/values/generics/image.rs
+++ b/components/style/values/generics/image.rs
@@ -108,7 +108,7 @@ pub use self::GenericCrossFadeImage as CrossFadeImage;
 #[css(comma, function = "image-set")]
 #[repr(C)]
 pub struct GenericImageSet<Image, Resolution> {
-    /// The index of the selected candidate. Zero for specified values.
+    /// The index of the selected candidate. usize::MAX for specified values or invalid images.
     #[css(skip)]
     pub selected_index: usize,
 
diff --git a/components/style/values/specified/image.rs b/components/style/values/specified/image.rs
index a9eb6bf6571..2a58d3f9ed6 100644
--- a/components/style/values/specified/image.rs
+++ b/components/style/values/specified/image.rs
@@ -399,7 +399,7 @@ impl ImageSet {
             })
         })?;
         Ok(Self {
-            selected_index: 0,
+            selected_index: std::usize::MAX,
             items: items.into(),
         })
     }