diff --git a/crates/stet-render/src/lib.rs b/crates/stet-render/src/lib.rs
index 4706ec5..1d774c3 100644
--- a/crates/stet-render/src/lib.rs
+++ b/crates/stet-render/src/lib.rs
@@ -43,6 +43,7 @@ pub use skia_device::ImageCache;
 pub use skia_device::PageBackground;
 pub use skia_device::PreparedDisplayList;
 pub use skia_device::RegionRender;
+pub use skia_device::VIEWPORT_TRANSPARENT;
 #[cfg(feature = "ps-device")]
 pub use skia_device::SkiaDevice;
 pub use skia_device::debug_bbox_comparison;
diff --git a/crates/stet-render/src/skia_device.rs b/crates/stet-render/src/skia_device.rs
index 8ecec42..a4e6a2b 100644
--- a/crates/stet-render/src/skia_device.rs
+++ b/crates/stet-render/src/skia_device.rs
@@ -2723,6 +2723,9 @@ fn build_clip_epochs(list: &DisplayList, bboxes: &[Option<YBBox>]) -> Vec<ClipEp
 /// The original transform maps from path space to full-page device space;
 /// we subtract `y_offset` from `ty` so band rows [y_start, y_start+band_h)
 /// map to pixmap rows [0, band_h).
+/// FormatYard: when set, viewport renders keep the page transparent (straight alpha) instead of white paper.
+pub static VIEWPORT_TRANSPARENT: std::sync::atomic::AtomicBool = std::sync::atomic::AtomicBool::new(false);
+
 /// Composite premultiplied-alpha RGBA pixels onto a white background.
 /// After this, all pixels are fully opaque (alpha=255).
 fn composite_onto_white(data: &mut [u8]) {
@@ -14099,8 +14102,12 @@ fn render_region_prepared_inner(
         }
     }
 
-    // Composite onto white background
-    composite_onto_white(pixmap.data_mut());
+    // Composite onto white background, or keep the page transparent (FormatYard: set_viewport_transparent)
+    if VIEWPORT_TRANSPARENT.load(std::sync::atomic::Ordering::Relaxed) {
+        unpremultiply(pixmap.data_mut());
+    } else {
+        composite_onto_white(pixmap.data_mut());
+    }
     // Extract only the requested pixel_h rows (skip the OVERLAP padding at the bottom).
     let row_bytes = pixel_w as usize * 4;
     let end = pixel_h as usize * row_bytes;
diff --git a/crates/stet-wasm/src/lib.rs b/crates/stet-wasm/src/lib.rs
index 03f9356..94681e2 100644
--- a/crates/stet-wasm/src/lib.rs
+++ b/crates/stet-wasm/src/lib.rs
@@ -808,6 +808,12 @@ pub fn render_viewport(
 /// Compute the number of bands and band height for viewport banding.
 ///
 /// Returns a JS array `[num_bands, band_height]`.
+/// FormatYard: viewport renders keep the page transparent (true) or put it on white paper (false, the default).
+#[wasm_bindgen]
+pub fn set_viewport_transparent(on: bool) {
+    stet_render::VIEWPORT_TRANSPARENT.store(on, std::sync::atomic::Ordering::Relaxed);
+}
+
 #[wasm_bindgen]
 pub fn viewport_band_params(pixel_w: u32, pixel_h: u32) -> js_sys::Array {
     let (num_bands, band_h) = stet_render::viewport_band_count(pixel_w, pixel_h);