Moving this thread out of Flash Professional FAQs to Flash Pro - General.
It's not very common to "preload HTML".
If you're displaying the HTML in a StageWebView then you should be listening for "complete" event to fire off to know when the HTML loading is finished. You can keep your StageWebView off-screen until the complete event fires. You can't get "percent loaded" information from StageWebView so the animation should just be a generic loading graphic, like a spinning wheel. Once it's complete, move the StageWebView into view. That will give you the appearance of a preloader on the HTML.