diff --git a/components/style/animation.rs b/components/style/animation.rs
index c69aca08bde..6eae69e8183 100644
--- a/components/style/animation.rs
+++ b/components/style/animation.rs
@@ -355,7 +355,15 @@ impl PropertyAnimation {
             TransitionTimingFunction::Frames(frames) => {
                 // https://drafts.csswg.org/css-timing/#frames-timing-functions
                 let mut out = (time * (frames as f64)).floor() / ((frames - 1) as f64);
-                if out > 1.0 && time <= 1.0 {
+                if out > 1.0 {
+                    // FIXME: Basically, during the animation sampling process, the input progress
+                    // should be in the range of [0, 1]. However, |time| is not accurate enough
+                    // here, which means |time| could be larger than 1.0 in the last animation
+                    // frame. (It should be equal to 1.0 exactly.) This makes the output of frames
+                    // timing function jumps to the next frame/level.
+                    // However, this solution is still not correct because |time| is possible
+                    // outside the range of [0, 1] after introducing Web Animations. We should fix
+                    // this problem when implementing web animations.
                     out = 1.0;
                 }
                 out