Okay, thank you!
The problem is exactly what I was suspecting. I looked at the page source and some CSS is printed as inline into the site header – which is all right, it should not cause the flash. However there is lots of other plugin CSS in the site footer. I see rev slider and other plugin CSS there as well.
If you look closely you can also see the logo and the button styles flash if empty the browser cache.
The cause might be a cache or CSS minification plugin possibly. Pushing CSS into the site footer makes your site faster, however it may also cause a flash of unstyled content. Try to re-configure the cache/minification plugin to print the CSS to the site header, it should solve this problem.Best,
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