Turns out defaultdict and str.format(**d) don't work well together; using string.Formatter().vformat() instead. My bad.
[youtube] Use the first v= argument in the URL
This is according to how youtube handles multiple v= values in one URL. Before this, it was possible to make a single URL show up differently on youtube itself, and if you downloaded/viewed it with youtube-dl/mpv
Improve generic SMIL support