Earlier quoted context omitted.
It's Youtube, not high frequency trading.
The logic is apparently that we can't do progressive enhancement because it would force us to make a second network request, but we can rely on unstandardized APIs that force us to ship polyfills that make our site slow on every other browser that's not Chrome. Every decision, from progressive enhancement to supporting bleeding-edge features comes with tradeoffs. Youtube is still choosing tradeoffs, it's just choosin…
Speaking personally from my time before working at Google, the second network request is definitely the worse of the two options if you can’t do anything until it completes for most people- the round trip time kills you in the median US case I am familiar with.
At Apple for my small project we didn’t have these constraints so we just shipped a giant JavaScript bundle with feature detection and browser-side re-rendering that worked alright.