On seemingly all AMP pages, the Safari Technology Preview browser displays a completely blank page for around 8 seconds. (Presumable content only becomes visible at all because the AMP boilerplate kicks in.)

For example:

• https://www.ampproject.org
• http://www.bbc.co.uk/news/amp/science-environment-36780756

Loading these pages with #development=1 appended to the URL indicates that the pages are valid AMP.