diff --git a/components/style/values/specified/gecko.rs b/components/style/values/specified/gecko.rs
index 3e3085c8849..9b01cc2a6c4 100644
--- a/components/style/values/specified/gecko.rs
+++ b/components/style/values/specified/gecko.rs
@@ -49,6 +49,11 @@ impl Parse for IntersectionObserverRootMargin {
         context: &ParserContext,
         input: &mut Parser<'i, 't>,
     ) -> Result<Self, ParseError<'i>> {
+        use crate::Zero;
+        if input.is_exhausted() {
+            // If there are zero elements in tokens, set tokens to ["0px"].
+            return Ok(IntersectionObserverRootMargin(Rect::all(LengthPercentage::zero())));
+        }
         let rect = Rect::parse_with(context, input, parse_pixel_or_percent)?;
         Ok(IntersectionObserverRootMargin(rect))
     }