diff --git a/Cargo.toml b/Cargo.toml
index 3ddcf9e..2a7f490 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -156,3 +156,19 @@ opt-level = 3
 [profile.release]
 opt-level = 3
 lto = "thin"
+
+# Size-focused profile for the browser canvas build (solitaire_web →
+# canvas_bg.wasm). Download size is the constraint on the web, not peak
+# throughput: fat LTO + one codegen unit + opt-level "s" cut the Bevy wasm
+# bundle substantially versus plain release. This is rustc-side sizing only —
+# the binaryen pass in build_wasm.sh stays at wasm-opt -O2 because -Oz has
+# miscompiled Bevy's render pipeline before (grey screen on first load).
+[profile.wasm-release]
+inherits = "release"
+opt-level = "s"
+lto = "fat"
+codegen-units = 1
+# No `strip`: on wasm it also removes the target_features custom section,
+# which makes wasm-opt reject the module ("all used features should be
+# allowed" on trunc_sat). wasm-opt drops the name section in its output
+# anyway, so strip buys nothing here.
diff --git a/build_wasm.sh b/build_wasm.sh
index 995d17d..cff37ab 100755
--- a/build_wasm.sh
+++ b/build_wasm.sh
@@ -67,7 +67,9 @@ if ! command -v wasm-bindgen &> /dev/null; then
 fi
 
 echo "Building solitaire_web (Bevy WASM app)..."
-cargo build --release --target wasm32-unknown-unknown -p solitaire_web
+# wasm-release is the size-focused profile (fat LTO, CGU=1, opt-level "s") —
+# see [profile.wasm-release] in Cargo.toml. Download size is the constraint.
+cargo build --profile wasm-release --target wasm32-unknown-unknown -p solitaire_web
 
 echo "Running wasm-bindgen for solitaire_web..."
 wasm-bindgen \
@@ -75,7 +77,7 @@ wasm-bindgen \
     --out-name canvas \
     --target web \
     --no-typescript \
-    "$REPO_ROOT/target/wasm32-unknown-unknown/release/solitaire_web.wasm"
+    "$REPO_ROOT/target/wasm32-unknown-unknown/wasm-release/solitaire_web.wasm"
 
 # Optional size optimisation — Bevy bundles are large (~5-15 MB uncompressed).
 # wasm-opt passes are skipped silently when the tool is not installed.