Pages I had years ago still load but videos won’t play on many of the platforms.
Big outfits like YouTube will serve out webpages to operate properly on all the newest platforms. Those of us with simple html pages are left in the dust. It could be an iphone or ipad or Android or a slew of others. There’s gotta be programs that create code that senses the browser platform and make the pages and videos created serve properly.
On video files, might be
.avi
.mp4
.ogv
Audio might be
.mp3
.ac3
.aac
Much of the video/audio codec confusion is due to lawsuits and copyrights. Tablet manuf have to make things different or they get sued. In turn the website creators have a mess to deal with.
I don’t think you can go wrong with mp3 and mp4.