diff --git a/pyproject.toml b/pyproject.toml
index 0d4ff924..cff92f9d 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -25,12 +25,14 @@ keywords = ["eeg", "preprocessing", "EEGLAB"]
 dependencies = [
   "colorama>=0.4.6",
   "eeglabio>=0.1.2",
-  # The h5py, matplotlib, numpy and threadpoolctl floors here, and the scipy
-  # floors below, are capped by what Pyodide 0.29.5 ships for a browser install.
-  # Raising one past Pyodide's shipped version breaks eegprep in the browser, so
-  # check against the Pyodide release before bumping any of them.
-  # Shipped by 0.29.5: h5py 3.13.0, matplotlib 3.8.4, numpy 2.2.5, scipy 1.14.1,
-  # threadpoolctl 3.5.0. tests/test_browser_dependency_floors.py enforces this.
+  # h5py, matplotlib and numpy here, and scipy below, publish no pure-Python wheel,
+  # so in a browser only Pyodide's bundled build is available and micropip cannot
+  # substitute another version. Their floors are therefore a ceiling: raising one
+  # past what Pyodide ships makes the install impossible, not merely larger.
+  # Shipped by 0.29.5: h5py 3.13.0, matplotlib 3.8.4, numpy 2.2.5, scipy 1.14.1.
+  # threadpoolctl is bundled too but is pure Python, so micropip can fetch any
+  # version from PyPI and its floor is not constrained this way.
+  # tests/test_browser_dependency_floors.py enforces the distinction.
   "h5py>=3.12.1",
   "matplotlib>=3.8.0",
   "mne>=1.10.0",
diff --git a/tests/test_browser_dependency_floors.py b/tests/test_browser_dependency_floors.py
index 6405c995..c2608cb0 100644
--- a/tests/test_browser_dependency_floors.py
+++ b/tests/test_browser_dependency_floors.py
@@ -30,14 +30,26 @@
 # the version and this table together when the target release moves.
 PYODIDE_VERSION = "0.29.5"
 PYODIDE_PYTHON = "3.13.2"
-PYODIDE_SHIPS = {
+
+# Packages Pyodide bundles that publish NO pure-Python wheel on PyPI. micropip cannot substitute
+# a different version for these, so the bundled build is the only one a browser can have and the
+# declared constraint has to be satisfiable by it. This is the set that makes a floor a ceiling.
+PYODIDE_COMPILED = {
     "h5py": "3.13.0",
     "matplotlib": "3.8.4",
     "numpy": "2.2.5",
     "scipy": "1.14.1",
+}
+
+# Also bundled, but pure Python, so micropip can pull any version straight from PyPI and a floor
+# above the bundled one costs an extra download rather than breaking the install. Recorded so the
+# distinction is explicit, and deliberately not enforced.
+PYODIDE_PURE_PYTHON = {
     "threadpoolctl": "3.5.0",
 }
 
+PYODIDE_SHIPS = {**PYODIDE_COMPILED, **PYODIDE_PURE_PYTHON}
+
 # A packaging marker environment describing Pyodide's interpreter, so a dependency guarded by
 # a marker is evaluated the way the browser would evaluate it. Pyodide reports
 # ``sys.platform == "emscripten"``, which is what makes the darwin-only scipy floor inactive here.
@@ -79,26 +91,26 @@ def _declared_floor(requirement: Requirement) -> Version:
     return max(floors)
 
 
-@pytest.mark.parametrize("package", sorted(PYODIDE_SHIPS))
+@pytest.mark.parametrize("package", sorted(PYODIDE_COMPILED))
 def test_floor_does_not_exceed_what_pyodide_ships(package: str) -> None:
     requirement = _requirements_active_in_pyodide().get(package)
-    assert requirement is not None, f"{package} is no longer a base dependency; drop it from PYODIDE_SHIPS"
+    assert requirement is not None, f"{package} is no longer a base dependency; drop it from PYODIDE_COMPILED"
 
     floor = _declared_floor(requirement)
-    shipped = Version(PYODIDE_SHIPS[package])
+    shipped = Version(PYODIDE_COMPILED[package])
     assert floor <= shipped, (
         f"{package}>={floor} is above the {shipped} that Pyodide {PYODIDE_VERSION} ships, "
         f"so a browser install cannot satisfy it"
     )
 
 
-@pytest.mark.parametrize("package", sorted(PYODIDE_SHIPS))
+@pytest.mark.parametrize("package", sorted(PYODIDE_COMPILED))
 def test_pyodide_version_satisfies_the_whole_specifier(package: str) -> None:
     """The floor is not the only bound; an upper bound can exclude Pyodide's build too."""
     requirement = _requirements_active_in_pyodide().get(package)
-    assert requirement is not None, f"{package} is no longer a base dependency; drop it from PYODIDE_SHIPS"
+    assert requirement is not None, f"{package} is no longer a base dependency; drop it from PYODIDE_COMPILED"
 
-    shipped = PYODIDE_SHIPS[package]
+    shipped = PYODIDE_COMPILED[package]
     assert requirement.specifier.contains(shipped, prereleases=True), (
         f"Pyodide {PYODIDE_VERSION} ships {package} {shipped}, which {requirement} excludes"
     )
@@ -124,7 +136,7 @@ def test_the_darwin_scipy_floor_stays_out_of_the_browser() -> None:
         and requirement.marker.evaluate(darwin)
     ]
     assert darwin_floors, "the darwin-specific scipy floor is gone; update or remove this test"
-    assert max(darwin_floors) > Version(PYODIDE_SHIPS["scipy"]), (
+    assert max(darwin_floors) > Version(PYODIDE_COMPILED["scipy"]), (
         "the darwin scipy floor no longer exceeds Pyodide's build, so the marker split may be "
         "unnecessary; confirm before removing this test"
     )
@@ -133,5 +145,5 @@ def test_the_darwin_scipy_floor_stays_out_of_the_browser() -> None:
         for raw in _project()["dependencies"]
         if (requirement := Requirement(raw)).name == "scipy"
         and requirement.marker is not None
-        and _declared_floor(requirement) > Version(PYODIDE_SHIPS["scipy"])
+        and _declared_floor(requirement) > Version(PYODIDE_COMPILED["scipy"])
     ), "a scipy floor above Pyodide's build is active under Pyodide"