diff --git a/src/renderer/RenderDevice.cpp b/src/renderer/RenderDevice.cpp
index 1fedf4e..6927546 100644
--- a/src/renderer/RenderDevice.cpp
+++ b/src/renderer/RenderDevice.cpp
@@ -1501,7 +1501,11 @@ RenderDevice::RenderDevice(
    SetRenderState(RenderState::ZFUNC, RenderState::Z_LESSEQUAL);
 
    // Retrieve a reference to the back buffer.
-   wnd->SetBackBuffer(new RenderTarget(this, SurfaceType::RT_DEFAULT, wnd->GetWidth(), wnd->GetHeight(), back_buffer_format));
+   // vpinball-wasm: use pixel (device) size, not logical/CSS size - on Emscripten
+   // these differ by devicePixelRatio, and the GL viewport (RenderTarget.cpp's
+   // glViewport(0, 0, m_width, m_height)) must match the canvas's actual backing
+   // buffer or rendering only fills a fraction of it, anchored bottom-left.
+   wnd->SetBackBuffer(new RenderTarget(this, SurfaceType::RT_DEFAULT, wnd->GetPixelWidth(), wnd->GetPixelHeight(), back_buffer_format));
 
 #elif defined(ENABLE_DX9)
    ///////////////////////////////////
@@ -2168,10 +2172,19 @@ void RenderDevice::WaitForVSync(const bool asynchronous)
       m_vsyncCount++;
       m_presentTimestampReference = usec();
    };
+#ifndef __EMSCRIPTEN__
    if (asynchronous)
       std::thread(lambda).detach(); // Reuse thread ? (we always at most one running at a time)
    else
       lambda();
+#else
+   // vpinball-wasm: a single-threaded Emscripten build (no -pthread /
+   // SharedArrayBuffer) cannot construct real std::thread workers - run
+   // synchronously inline instead. This only updates m_vsyncCount/timestamp
+   // bookkeeping (the real vblank-wait branches above are already excluded
+   // for __STANDALONE__ builds), so synchronous execution is equivalent.
+   lambda();
+#endif
 }
 
 #if defined(ENABLE_BGFX)