diff --git a/.cargo/config.toml b/.cargo/config.toml
new file mode 100644
index 00000000..dd11e687
--- /dev/null
+++ b/.cargo/config.toml
@@ -0,0 +1,29 @@
+# getrandom 0.3 refuses to build for wasm32-unknown-unknown unless it is told
+# which backend to use. The target has no OS entropy source, so the crate will
+# not guess: it wants the `wasm_js` backend named explicitly, AND the matching
+# feature enabled on the crate itself. Its own error is unusually clear that one
+# without the other is not enough:
+#
+#   error: The wasm32-unknown-unknown targets are not supported by default; you
+#   may need to enable the "wasm_js" configuration flag. Note that enabling the
+#   `wasm_js` feature flag alone is insufficient.
+#
+# This is the cfg half. The feature half is a wasm32-only dependency in
+# crates/sylpheed-viewer/Cargo.toml — that is the only crate reaching getrandom
+# here, transitively through `ahash`.
+#
+# Scoped to the wasm target, so native builds are untouched. Note that a
+# RUSTFLAGS environment variable, if one is ever set, replaces this rather than
+# adding to it.
+#
+# The second cfg is bevy_egui's. Its web clipboard support calls web-sys APIs
+# that are still gated behind an unstable flag, and it refuses to build without
+# it rather than silently dropping the feature:
+#
+#   error: bevy_egui uses unstable APIs to support clipboard on web.
+#
+[target.wasm32-unknown-unknown]
+rustflags = [
+    '--cfg', 'getrandom_backend="wasm_js"',
+    '--cfg', 'web_sys_unstable_apis',
+]
diff --git a/Cargo.lock b/Cargo.lock
index 83cbd322..2c962ed3 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2497,9 +2497,11 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "899def5c37c4fd7b2664648c28120ecec138e4d395b459e5ca34f9cce2dd77fd"
 dependencies = [
  "cfg-if",
+ "js-sys",
  "libc",
  "r-efi",
  "wasip2",
+ "wasm-bindgen",
 ]
 
 [[package]]
@@ -4675,6 +4677,7 @@ dependencies = [
  "bevy",
  "bevy_egui",
  "futures",
+ "getrandom 0.3.4",
  "image",
  "rfd",
  "rodio",
diff --git a/crates/sylpheed-viewer/Cargo.toml b/crates/sylpheed-viewer/Cargo.toml
index 455938d6..3a3816e5 100644
--- a/crates/sylpheed-viewer/Cargo.toml
+++ b/crates/sylpheed-viewer/Cargo.toml
@@ -56,3 +56,10 @@ rodio              = { workspace = true }
 image              = { version = "0.25", default-features = false, features = ["png"] }
 # Off-thread rasterization of embedded-font samples (avoids egui's global fonts).
 ab_glyph           = "0.2"
+
+# Reached only through `ahash`, which needs an entropy source for its random
+# state. Declared here purely to turn on the `wasm_js` feature; nothing in this
+# crate calls getrandom directly. The paired `--cfg getrandom_backend` lives in
+# /.cargo/config.toml — both are required, neither is sufficient.
+[target.'cfg(target_arch = "wasm32")'.dependencies]
+getrandom = { version = "0.3", features = ["wasm_js"] }