diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5d7e84c48c..a0d8fb012b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -23,6 +23,7 @@ avifEncoder struct.
   minQuantizer, maxQuantizer, minQuantizerAlpha, and maxQuantizerAlpha
   initialized to the default values.
 * Add the public API function avifImageIsOpaque() in avif.h.
+* Add experimental API for progressive AVIF encoding.
 
 ### Changed
 * Exif and XMP metadata is exported to PNG and JPEG files by default,
diff --git a/include/avif/avif.h b/include/avif/avif.h
index 136b9b8e74..f8c9a73c14 100644
--- a/include/avif/avif.h
+++ b/include/avif/avif.h
@@ -1117,13 +1117,13 @@ typedef struct avifEncoder
     // settings (see Notes above)
     int maxThreads;
     int speed;
-    int keyframeInterval; // How many frames between automatic forced keyframes; 0 to disable (default).
-    uint64_t timescale;   // timescale of the media (Hz)
-    int repetitionCount;  // Number of times the image sequence should be repeated. This can also be set to
-                          // AVIF_REPETITION_COUNT_INFINITE for infinite repetitions.  Only applicable for image sequences.
-                          // Essentially, if repetitionCount is a non-negative integer `n`, then the image sequence should be
-                          // played back `n + 1` times. Defaults to AVIF_REPETITION_COUNT_INFINITE.
-    uint32_t extraLayerCount;
+    int keyframeInterval;     // How many frames between automatic forced keyframes; 0 to disable (default).
+    uint64_t timescale;       // timescale of the media (Hz)
+    int repetitionCount;      // Number of times the image sequence should be repeated. This can also be set to
+                              // AVIF_REPETITION_COUNT_INFINITE for infinite repetitions.  Only applicable for image sequences.
+                              // Essentially, if repetitionCount is a non-negative integer `n`, then the image sequence should be
+                              // played back `n + 1` times. Defaults to AVIF_REPETITION_COUNT_INFINITE.
+    uint32_t extraLayerCount; // EXPERIMENTAL: Non-zero value encodes layered image.
 
     // changeable encoder settings
     int quality;