Never mind. Looks like I needed to add the -vcodec parameter.
var params = [
'-i', 'pipe:0', // Tell avconv to expect an input stream (via its stdin)
In this review, RPi 4 was able to play a 1080p30 Youtube video smoothly, but not 1080p60. There was observable screen tearing, but I'd expect the situation to improve in this regard as Raspbian is polished for RPi 4 support.
Note that there's also a method to offload the video playback from the browser to an external video player, which sometimes was the ...