So, it's hard to understand exactly what it is you're trying to accomplish and why Web Workers wouldn't work for you because basically every single desktop GUI framework is single-threaded when it comes to actually updating the components on the screen. Win32 uses the GetMessage/TranslateMessage/DispatchMessage event loop. Swing has the UI thread, and then to update the UI from a background thread, you need to call invokeAndWait or invokeLater and pass a Runnable. Cocoa and iOS have Grand Central Dispatch, and to update the UI you pass a block to dispatch_async on a dispatch queue. Android has you call View.post() with a Runnable that contains your UI modifications. WPF has two threads, a rendering one and a UI thread, and you're supposed to pass long-running operations off to a threadpool and then call Invoke or BeginInvoke with a delegate to actually update the UI from them.

Web Workers are exactly this model, applied to the web. It's a dispatch queue running inside a threadpool. To update the DOM, you postMessage back to the UI thread and make your modifications there.

Are you willing to let a stranger on the Internet change your opinion?