Well not all platforms are created equal... it's why you have fall-backs to "Stable but not shiny" for platforms you don't support... or platforms you don't know supports New Shiney Thing (tm).
If you know browser X, Y and Z support HTML5 video - because that's your actively supported platforms... but someone says I'm browser 6... do you push HTML5 video? Or do you fall back to stuff you know works but is years old tech?
If you don't know what Browser 6 is... then you have 2 options: Safe options or Feature Detection.
Safe option: I don't know if you can read HTML5... so I'm going to serve you HTML like it's 1999.
Feature Detection: I'm going to go out of my way to see if you can support random new technologies that I'm using. Can use HTML5? You can? Oh... Here's your scripts... or... You can't? Here's HTML like it's 1999.
The first option is a lot easier. "Unknown? You get 'Safe' tech that's not shiny, optimized or best-of-breed. Known? You get stuff we said we support on platforms we say we support".
Second option requires testing and blending the technology stack based on detected features. Possible - because plenty of people do it - but it's definitely not as easy to support dozens of techs in many many more possible combinations.
It looks like MS took Door #1.
Are you saying you would ship HTML5 code to platforms you don't know support HTML5? Because... that sounds like a recipe to send "broken" code to users because "everything should work the same everywhere" which is something that happens... never.