Fix handling of negative numbers in list counters.

Fixes #6818.

components/layout/generated_content.rs

@@ -522,37 +522,57 @@ pub fn static_representation(list_style_type: list_style_type::T) -> char {
 /// Pushes the string that represents the value rendered using the given *alphabetic system* onto
 /// the accumulator per CSS-COUNTER-STYLES § 3.1.4.
 fn push_alphabetic_representation(mut value: i32, system: &[char], accumulator: &mut String) {
+    let mut abs_value = handle_negative_value(value, accumulator);
+
     let mut string: SmallVec<[char; 8]> = SmallVec::new();
-    while value != 0 {
+    while abs_value != 0 {
         // Step 1.
-        value = value - 1;
+        abs_value = abs_value - 1;
         // Step 2.
-        string.push(system[(value as usize) % system.len()]);
+        string.push(system[abs_value % system.len()]);
         // Step 3.
-        value = ((value as usize) / system.len()) as i32;
+        abs_value = abs_value / system.len();
     }
 
     accumulator.extend(string.iter().cloned().rev())
 }
 
 /// Pushes the string that represents the value rendered using the given *numeric system* onto the
-/// accumulator per CSS-COUNTER-STYLES § 3.1.4.
+/// accumulator per CSS-COUNTER-STYLES § 3.1.5.
 fn push_numeric_representation(mut value: i32, system: &[char], accumulator: &mut String) {
+    let mut abs_value = handle_negative_value(value, accumulator);
+
     // Step 1.
-    if value == 0 {
+    if abs_value == 0 {
         accumulator.push(system[0]);
         return
     }
 
     // Step 2.
     let mut string: SmallVec<[char; 8]> = SmallVec::new();
-    while value != 0 {
+    while abs_value != 0 {
         // Step 2.1.
-        string.push(system[(value as usize) % system.len()]);
+        string.push(system[abs_value % system.len()]);
         // Step 2.2.
-        value = ((value as usize) / system.len()) as i32;
+        abs_value = abs_value / system.len();
     }
 
     // Step 3.
     accumulator.extend(string.iter().cloned().rev())
 }
+
+/// If the system uses a negative sign, handle negative values per CSS-COUNTER-STYLES § 2.
+///
+/// Returns the absolute value of the counter.
+fn handle_negative_value(value: i32, accumulator: &mut String) -> usize {
+    // 3. If the counter value is negative and the counter style uses a negative sign, instead
+    //    generate an initial representation using the absolute value of the counter value.
+    if value < 0 {
+        // TODO: Support different negative signs using the 'negative' descriptor.
+        // https://drafts.csswg.org/date/2015-07-16/css-counter-styles/#counter-style-negative
+        accumulator.push('-');
+        value.abs() as usize
+    } else {
+        value as usize
+    }
+}