style: Support rendering content: <gradient> images

We implemented support for list-style-image anyways.

Differential Revision: https://phabricator.services.mozilla.com/D172343

components/style/values/generics/counters.rs

@@ -237,7 +237,7 @@ impl<Image> Content<Image> {
 
 /// Items for the `content` property.
 #[derive(
-    Clone, Debug, Eq, MallocSizeOf, PartialEq, ToComputedValue, ToCss, ToResolvedValue, ToShmem,
+    Clone, Debug, Eq, MallocSizeOf, PartialEq, ToComputedValue, SpecifiedValueInfo, ToCss, ToResolvedValue, ToShmem,
 )]
 #[repr(u8)]
 pub enum GenericContentItem<I> {

components/style/values/specified/counters.rs

@@ -17,7 +17,7 @@ use crate::values::specified::Integer;
 use crate::values::CustomIdent;
 use cssparser::{Parser, Token};
 use selectors::parser::SelectorParseErrorKind;
-use style_traits::{KeywordsCollectFn, ParseError, SpecifiedValueInfo, StyleParseErrorKind};
+use style_traits::{ParseError, StyleParseErrorKind};
 
 #[derive(PartialEq)]
 enum CounterType {
@@ -201,7 +201,7 @@ impl Parse for Content {
         let mut has_alt_content = false;
         loop {
             {
-                if let Ok(image) = input.try_parse(|i| Image::parse_only_url(context, i)) {
+                if let Ok(image) = input.try_parse(|i| Image::parse_forbid_none(context, i)) {
                     content.push(generics::ContentItem::Image(image));
                     continue;
                 }
@@ -279,20 +279,3 @@ impl Parse for Content {
         Ok(generics::Content::Items(content.into()))
     }
 }
-
-impl<Image> SpecifiedValueInfo for generics::GenericContentItem<Image> {
-    fn collect_completion_keywords(f: KeywordsCollectFn) {
-        f(&[
-            "url",
-            "image-set",
-            "counter",
-            "counters",
-            "attr",
-            "open-quote",
-            "close-quote",
-            "no-open-quote",
-            "no-close-quote",
-            "-moz-alt-content",
-        ]);
-    }
-}

components/style/values/specified/image.rs

@@ -291,6 +291,19 @@ impl Image {
         )
     }
 
+    /// Provides an alternate method for parsing, but forbidding `none`
+    pub fn parse_forbid_none<'i, 't>(
+        context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Image, ParseError<'i>> {
+        Self::parse_with_cors_mode(
+            context,
+            input,
+            CorsMode::None,
+            ParseImageFlags::FORBID_NONE
+        )
+    }
+
     /// Provides an alternate method for parsing, but only for urls.
     pub fn parse_only_url<'i, 't>(
         context: &ParserContext,