diff --git a/.gitea/workflows/ci.yml b/.gitea/workflows/ci.yml
index 1a7cdde..853a729 100644
--- a/.gitea/workflows/ci.yml
+++ b/.gitea/workflows/ci.yml
@@ -22,6 +22,9 @@ jobs:
         run: rustup component add rustfmt clippy
       - name: Install thumbv7em-none-eabihf target
         run: rustup target add thumbv7em-none-eabihf
+      - name: Install wasm32-unknown-unknown target
+        # ci-test.sh builds the reader and clawhdf5-wasm for the browser.
+        run: rustup target add wasm32-unknown-unknown
       - name: Install Python interop dependencies
         # The interop suites used to skip silently when python3/h5py were
         # missing, so they never ran in CI. Install them and make a missing
diff --git a/scripts/ci-test.sh b/scripts/ci-test.sh
index 1d7a7f9..6cad171 100755
--- a/scripts/ci-test.sh
+++ b/scripts/ci-test.sh
@@ -106,6 +106,14 @@ no_c_in_default_build() {
 }
 run_step "no C in the default build (core crates)" no_c_in_default_build
 
+# The reader in the browser: the facade's read path must build for
+# wasm32-unknown-unknown (no mmap, no threads, no file system). Needs
+# `rustup target add wasm32-unknown-unknown`.
+run_step "wasm32 build (clawhdf5, no default features)" cargo build \
+    -p clawhdf5 \
+    --target wasm32-unknown-unknown \
+    --no-default-features
+
 # The workspace declares a minimum Rust version (rust-version in Cargo.toml);
 # check that it really builds there, so the README badge and the manifests
 # cannot drift from the truth. Separate target dir: a different toolchain