Set Flash Player to auto update in the Adobe Flash Player preference pane.
You have to allow Flash Player on the site as Safari users are sent an acient video player (I found that it reads macOS 10.13 and higher as 10.1 (reported that to Comcast about 3 years ago).
Other browser get a sitghtly broken HTML5 video player (Chroiumum DRM spikes as much as the website tab playing the content)
Should note that Comcast has less than a few months before browsers remove Adobe Flash Player support and Adobe has a EOL date for Flash Player near end of the year.
I can stream 1080 HD and 4k on other sites with no CPU spike but I can't even stream 360 resoution without a 80%+ CPU spike on the site due to broken old code. The code on the xfinity site runs like it was last updated for IE6 in about 2004.
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@Rustyben 100% Flash Player is being used in the video player, right now it runs like Flash Player code written in the mid 2000 (full updated Flash Player) CPU spikes of around 70 to 100+% CPU just for the plug in for low 360 resolution live video stream (I have done 4k resolution live streams and the CPU usage is around 20% or lower). Comcast looks to be using code that sees macOS 10.13+ as Mac OS X 10.1 (I reported this over 2 years ago to Comcast).
Strange that for Safari on desktop is not getting the same DRM and video player that is used in Comcast's iOS app.
If you mask as Chrome with a Choimum based browser you look to get an exterimental HTML5 video player and it using Google's Widevine for DRM.
The protocol used for the streams is RTMPE and for some devices RTMPTE. both encrypt the TCP traffic from one end to the other preventing any man-in-the-middle decrypted stream. That is the current security component while others are not being tested as possible replacements. 1080p (1K) and higher are requiring more and more security to prevent theft of the video by pirates, etc.
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