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@zalex108
Nope. Doesn't help. Guess I have no choice but come back to Win 7.
Windows 8.1 WMP doesn't have the right support that Opera needs for some reason.
Are you sure about this ? Because I ask this trouble on a local IT forum too and 2 of repliers said things were normal for them on Win 8.1.
@leocg
Just did what you said. Uninstalled the K-Lite Codec pack from my laptop.
Then I downloaded a sample video from Vimeo.com. No, WMP can't play it properly.
It still runs, there are sounds but no images.
@zalex108
Tried it, still no luck.
@zalex108
It worked perfectly on 2 days ago, when I was on Win 7. The problem happened when I moved to Win 8.
Yes, Opera Dev has the same result.
1/ Nope. My Windows has WMP.
2/ Yes, I've tried that.
3/ Same as 2/
4/ No. YouTube says my browser doesn't support H.264 and MSE & H.264.
5/ The video at bitmovin plays. They says "MSE is supported by your current browser." "EME is not supported by your current browser."
6/ Opera Developer doesn't help.
7/ Except YouTube, I can't play video any elsewhere, including Udemy, dailymotion, vimeo, ZingMp3,...
I've tried updating the browser, turn off extensions, update to the latest version of Adobe Flash player but nothing helps. 
The only video site I could watch is YouTube. Those videos work perfect on Chrome though. I use Windows 8.1 Pro 64 bit.