The threaded browser build (web/Dockerfile, ADR 64, ADR 66): the page may
ask for fewer threads than the browser has cores, in the environment
variable OPENSCAD_THREADS, which caps oneTBB for the whole run.

--- a/src/main_wrapper.cc
+++ b/src/main_wrapper.cc
@@ -1,5 +1,13 @@
 #include "openscad.h"
 
+#ifdef __EMSCRIPTEN__
+#include <cstdlib>
+#include <optional>
+#ifdef ENABLE_TBB
+#include <oneapi/tbb/global_control.h>
+#endif
+#endif
+
 #ifndef OPENSCAD_NOGUI
 #include <QtCore/qresource.h>  // Bring in Q_INIT_RESOURCE
 #endif
@@ -19,6 +27,15 @@ int main(int argc, char **argv)
   Q_INIT_RESOURCE(mac);
 #endif
 #endif
+#if defined(__EMSCRIPTEN__) && defined(ENABLE_TBB)
+  // At most OPENSCAD_THREADS threads, where the page asks for fewer.
+  static std::optional<oneapi::tbb::global_control> threads;
+  if (const char *limit = std::getenv("OPENSCAD_THREADS")) {
+    if (const int n = std::atoi(limit); n > 0) {
+      threads.emplace(oneapi::tbb::global_control::max_allowed_parallelism, static_cast<size_t>(n));
+    }
+  }
+#endif
   return openscad_main(argc, argv);
 }