With what I've been taught , I still don't see how the video tag in html 5 is showing that html could replace Js. The video tag is pretty limited in what it can do, and you still need DOM reference (therefore script) to add more functionality.
https://www.w3schools.com/tags/ref_av_dom.asp
A video is still an external element linked to the webpage,
http://img.techpowerup.org/170918/webv.jpg
I really don't see how you can compare video playback to being able to add, modify on the fly element (HTLM DOM) of the webpage.
Flash was a plug-in , but Js isn't a plug-in, like css it's a language that was made to communicate with hmtl and extend what you can do with a webpage (it can do more but that's what it's more used for)
With the way thing are working now If I want to make a slideshow that would have the same behavior on x page and another behavior in xx page, i can just use JS/Jquery and write once all the parameters that i want on an external .js file, and I'm done. If they added a slideshow tag in html, i would have to copy paste those parameters on everysingle webpage, that's as impraticable as writing the css into the html page.