diff --git a/components/style/gecko/media_queries.rs b/components/style/gecko/media_queries.rs
index 08ef628f1ff..5d960a8e9e1 100644
--- a/components/style/gecko/media_queries.rs
+++ b/components/style/gecko/media_queries.rs
@@ -432,4 +432,11 @@ impl Device {
         };
         SideOffsets2D::new(top, right, bottom, left)
     }
+
+    /// Returns true if the given MIME type is supported
+    pub fn is_supported_mime_type(&self, mime_type: &str) -> bool {
+        unsafe {
+            bindings::Gecko_IsSupportedImageMimeType(mime_type.as_ptr(), mime_type.len() as u32)
+        }
+    }
 }
diff --git a/components/style/values/computed/image.rs b/components/style/values/computed/image.rs
index 5e123a07abb..6290ac68a5d 100644
--- a/components/style/values/computed/image.rs
+++ b/components/style/values/computed/image.rs
@@ -74,9 +74,20 @@ impl ToComputedValue for specified::ImageSet {
     fn to_computed_value(&self, context: &Context) -> Self::ComputedValue {
         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_resolution = items[0].resolution.dppx();
-        for (i, item) in items.iter().enumerate().skip(1) {
+
+        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) {
+                continue;
+            }
+
             let candidate_resolution = item.resolution.dppx();
 
             // https://drafts.csswg.org/css-images-4/#image-set-notation:
@@ -97,7 +108,9 @@ impl ToComputedValue for specified::ImageSet {
                 false
             };
 
-            if better_candidate() {
+            // The first item with a supported MIME type is obviously the current best candidate
+            if !supported_image || better_candidate() {
+                supported_image = true;
                 selected_index = i;
                 selected_resolution = candidate_resolution;
             }
diff --git a/components/style/values/generics/image.rs b/components/style/values/generics/image.rs
index d49a8a2b4de..014ae46db02 100644
--- a/components/style/values/generics/image.rs
+++ b/components/style/values/generics/image.rs
@@ -132,7 +132,7 @@ pub struct GenericImageSet<Image, Resolution> {
 
 /// An optional percent and a cross fade image.
 #[derive(
-    Clone, Debug, MallocSizeOf, PartialEq, ToComputedValue, ToResolvedValue, ToShmem, ToCss,
+    Clone, Debug, MallocSizeOf, PartialEq, ToComputedValue, ToResolvedValue, ToShmem,
 )]
 #[repr(C)]
 pub struct GenericImageSetItem<Image, Resolution> {
@@ -142,7 +142,33 @@ pub struct GenericImageSetItem<Image, Resolution> {
     ///
     /// TODO: Skip serialization if it is 1x.
     pub resolution: Resolution,
-    // TODO: type() function.
+
+    /// The `type(<string>)`
+    /// (Optional) Specify the image's MIME type
+    pub mime_type: crate::OwnedStr,
+
+    /// True if mime_type has been specified
+    pub has_mime_type: bool,
+}
+
+impl<I: style_traits::ToCss, R: style_traits::ToCss> ToCss for GenericImageSetItem<I, R>
+{
+    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: fmt::Write,
+    {
+        self.image.to_css(dest)?;
+        dest.write_str(" ")?;
+        self.resolution.to_css(dest)?;
+
+        if self.has_mime_type {
+            dest.write_str(" ")?;
+            dest.write_str("type(")?;
+            self.mime_type.to_css(dest)?;
+            dest.write_str(")")?;
+        }
+        Ok(())
+    }
 }
 
 pub use self::GenericImageSet as ImageSet;
diff --git a/components/style/values/specified/image.rs b/components/style/values/specified/image.rs
index 0be0fcd2a0c..85c178e064d 100644
--- a/components/style/values/specified/image.rs
+++ b/components/style/values/specified/image.rs
@@ -377,6 +377,13 @@ impl ImageSet {
 }
 
 impl ImageSetItem {
+    fn parse_type<'i>(p: &mut Parser<'i, '_>) -> Result<crate::OwnedStr, ParseError<'i>> {
+        p.expect_function_matching("type")?;
+        p.parse_nested_block(|input| {
+            Ok(input.expect_string()?.as_ref().to_owned().into())
+        })
+    }
+
     fn parse<'i, 't>(
         context: &ParserContext,
         input: &mut Parser<'i, 't>,
@@ -393,10 +400,20 @@ impl ImageSetItem {
                 context, input, cors_mode, /* allow_none = */ false,  /* only_url = */ only_url
             )?,
         };
-        let resolution = input
-            .try_parse(|input| Resolution::parse(context, input))
-            .unwrap_or(Resolution::X(1.0));
-        Ok(Self { image, resolution })
+
+        let mut resolution = input.try_parse(|input| Resolution::parse(context, input)).ok();
+        let mime_type = input.try_parse(Self::parse_type).ok();
+
+        // Try to parse resolution after type().
+        if mime_type.is_some() && resolution.is_none() {
+            resolution = input.try_parse(|input| Resolution::parse(context, input)).ok();
+        }
+
+        let resolution = resolution.unwrap_or(Resolution::X(1.0));
+        let has_mime_type = mime_type.is_some();
+        let mime_type = mime_type.unwrap_or_default();
+
+        Ok(Self { image, resolution, has_mime_type, mime_type })
     }
 }