diff --git a/Makefile b/Makefile
index 67000b141c..2b5eacda9f 100644
--- a/Makefile
+++ b/Makefile
@@ -696,6 +696,14 @@ compat-table: esbuild | compat-table/node_modules
 	./esbuild compat-table/src/index.ts --bundle --platform=node --external:./compat-table/repos/* --outfile=compat-table/out.js --log-level=warning --sourcemap
 	node --enable-source-maps compat-table/out.js
 
+check-compat-table: compat-table
+	@if ! git diff --quiet internal/compat/js_table.go internal/compat/css_table.go 2>/dev/null; then \
+		echo "ERROR: Compat tables are out of date. Run 'make compat-table' and commit the changes."; \
+		git diff --stat internal/compat/js_table.go internal/compat/css_table.go; \
+		exit 1; \
+	fi
+	@echo "Compat tables are up to date."
+
 update-compat-table: esbuild
 	cd compat-table && npm i @mdn/browser-compat-data@latest caniuse-lite@latest --silent --save-exact
 	./esbuild compat-table/src/index.ts --bundle --platform=node --external:./compat-table/repos/* --outfile=compat-table/out.js --log-level=warning --sourcemap
diff --git a/internal/compat/compat_test.go b/internal/compat/compat_test.go
index 8e1cbee598..11b6c06187 100644
--- a/internal/compat/compat_test.go
+++ b/internal/compat/compat_test.go
@@ -61,3 +61,18 @@ func TestCompareVersions(t *testing.T) {
 	check(v{1, 2, 3}, Semver{Parts: []int{1, 2, 3}, PreRelease: "-pre"}, '>')
 	check(v{1, 2, 2}, Semver{Parts: []int{1, 2, 3}, PreRelease: "-pre"}, '<')
 }
+
+func TestFeatureLimits(t *testing.T) {
+	// The bitmask for JSFeature is uint64, which means it can only hold up to 64 flags.
+	// If this limit is exceeded, compilation will fail due to the compile-time guard in
+	// js_table.go, but this test provides a clearer error message.
+	if len(jsTable) > 64 {
+		t.Errorf("JSFeature capacity exceeded (max 64, got %d)", len(jsTable))
+	}
+
+	// The bitmask for CSSFeature is uint16, which means it can only hold up to 16 flags.
+	if len(cssTable) > 16 {
+		t.Errorf("CSSFeature capacity exceeded (max 16, got %d)", len(cssTable))
+	}
+}
+
diff --git a/internal/compat/js_table.go b/internal/compat/js_table.go
index 0a4ff9c312..6d8b547b92 100644
--- a/internal/compat/js_table.go
+++ b/internal/compat/js_table.go
@@ -121,8 +121,17 @@ const (
 	TypeofExoticObjectIsObject
 	UnicodeEscapes
 	Using
+
+	// WARNING: JSFeature is a uint64 bitmask. As of this writing, 61 of 64 bits are
+	// used (only 3 remaining). If more features are needed, the type must be widened
+	// or the representation changed.
 )
 
+// Compile-time assertion: fails if the number of JSFeature values reaches 64.
+// When the last iota equals 63, Using would be 1<<63 and doubling it overflows
+// uint64, causing a compile error. When iota reaches 64, the shift overflows to 0.
+const _ = uint64(Using) * 2
+
 var StringToJSFeature = map[string]JSFeature{
 	"arbitrary-module-namespace-names":  ArbitraryModuleNamespaceNames,
 	"array-spread":                      ArraySpread,