diff --git a/apps/chuzz/src/capture.rs b/apps/chuzz/src/capture.rs
index 7fd9ae7..1d8b0c9 100644
--- a/apps/chuzz/src/capture.rs
+++ b/apps/chuzz/src/capture.rs
@@ -45,6 +45,29 @@ fn capture_scale() -> f32 {
         .unwrap_or(1.0)
 }
 
+/// CSS pixel size to lay the page out at. Defaults to 1440 by 960.
+///
+/// A capture is only comparable against a reference browser when both laid the
+/// page out at the same width: every responsive breakpoint, every percentage
+/// width and every centred box moves with it. These were exported by
+/// `scripts/render-check.sh` and read by nothing, so a run at another size
+/// silently produced the default and the resulting diff was all viewport and
+/// no signal.
+pub fn capture_viewport() -> (u32, u32) {
+    fn dimension(name: &str, fallback: u32) -> u32 {
+        std::env::var(name)
+            .ok()
+            .and_then(|value| value.parse().ok())
+            .filter(|pixels: &u32| *pixels > 0)
+            .unwrap_or(fallback)
+    }
+
+    (
+        dimension("CHUZZ_CAPTURE_WIDTH", 1440),
+        dimension("CHUZZ_CAPTURE_HEIGHT", 960),
+    )
+}
+
 /// Which colour scheme to render at. Defaults to dark.
 ///
 /// A site that respects `prefers-color-scheme` is a different page in each, so
diff --git a/apps/chuzz/src/tauri_main.rs b/apps/chuzz/src/tauri_main.rs
index 148c97d..2565453 100644
--- a/apps/chuzz/src/tauri_main.rs
+++ b/apps/chuzz/src/tauri_main.rs
@@ -186,10 +186,11 @@ fn main() {
             .enable_all()
             .build()
             .expect("capture needs a tokio runtime");
+        let (width, height) = capture::capture_viewport();
         let result = runtime.block_on(capture::capture(
             &url,
-            1440,
-            960,
+            width,
+            height,
             std::path::Path::new(&output),
         ));
         match result {