diff --git a/moli-renderer-v8/src/native_bridge/element/styles/declaration/parse.rs b/moli-renderer-v8/src/native_bridge/element/styles/declaration/parse.rs
index f5eab438af..8f8e523a20 100644
--- a/moli-renderer-v8/src/native_bridge/element/styles/declaration/parse.rs
+++ b/moli-renderer-v8/src/native_bridge/element/styles/declaration/parse.rs
@@ -4252,6 +4252,68 @@ mod tests {
         );
     }
 
+    #[test]
+    fn content_alt_counters_use_validated_supplemental_entries() {
+        for (input, expected) in [
+            ("\"\" / counter(cnt)", "\"\" / counter(cnt)"),
+            (
+                "\"regular text\" / \"alt text 1\" counter(cnt) \"alt text 2\"",
+                "\"regular text\" / \"alt text 1\" counter(cnt) \"alt text 2\"",
+            ),
+            (
+                "\"regular text\" / counter(cnt) \"alt text\"",
+                "\"regular text\" / counter(cnt) \"alt text\"",
+            ),
+            (
+                "\"regular text\" / counters(chapter, \".\", DECIMAL)",
+                "\"regular text\" / counters(chapter, \".\")",
+            ),
+            (
+                "\"main / label\" / /* alt */ counter(cnt)",
+                "\"main / label\" / counter(cnt)",
+            ),
+        ] {
+            let parsed = parse_style_property_entries_for_cssom_write("content", input, true, None)
+                .unwrap_or_else(|| panic!("content: {input} should parse"));
+            assert_eq!(parsed.entries.len(), 1, "content: {input}");
+            assert_eq!(parsed.entries[0].name, "content", "content: {input}");
+            assert_eq!(parsed.entries[0].value, expected, "content: {input}");
+            assert!(parsed.entries[0].priority, "content: {input}");
+            assert_eq!(parsed.affected_names, vec!["content".to_owned()]);
+            assert!(style_entry_is_pdb_safe(&parsed.entries[0]));
+            assert!(
+                style_entry_is_pdb_supplemental_side_entry(&parsed.entries[0]),
+                "content: {input} should use side storage until Stylo parses alt counters"
+            );
+        }
+
+        for invalid in [
+            "none / counter(cnt)",
+            "\"\" / counter()",
+            "\"\" / url(alt.svg) counter(cnt)",
+            "\"\" / open-quote counter(cnt)",
+            "\"\" / counter(cnt) / \"extra\"",
+            "\"\" / counter(cnt) }",
+        ] {
+            assert!(
+                parse_style_property_entries_for_cssom_write("content", invalid, false, None)
+                    .is_none(),
+                "content: {invalid} should remain invalid"
+            );
+        }
+
+        let ordinary = parse_style_property_entries_for_cssom_write(
+            "content",
+            "\"regular text\" / \"alt text\"",
+            false,
+            None,
+        )
+        .expect("ordinary string alt text should remain Stylo-backed");
+        assert!(!style_entry_is_pdb_supplemental_side_entry(
+            &ordinary.entries[0]
+        ));
+    }
+
     #[test]
     fn transition_pdb_parser_accepts_dynamic_numeric_longhands() {
         let shorthand = parse_style_property_entries_with_base(