Yesterday I found some spare time and I decided to upgrade my Windows 10 1151 TH2 (build 10586) to the new Windows 10 1607 Anniversary Edition. After the upgrade was complete, I loaded IE 11 as usual and browsed to mail.yahoo.gr to read my e-mail. I then saw the "classic" Yahoo interface and a warning message at the top of the page stating that I see that because my browser is not supported and I should download and install another browser! This is exactly what one sees when opening Yahoo mail from a browser such as IE 8, with the important exception I was using IE 11! Immediately I thought I had some compatibility setting for Yahoo screwing the modern interface and resorting to the classic. I opened the Compatibility View, there was nothing on the list, until I noticed a check box in "Use Microsoft Compatibility lists". I tried to uncheck it and reload Yahoo mail, that was it! Now Yahoo Mail appears in the modern interface as it should! So if you experience similar symptoms in a webpage, try unchecking that box, see photo below:

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When I have more time and examine the new features or bugs of Windows 10 Anniversary Edition, I'll post again!