ctrl-l is commonly "focus on browser address bar".
ctrl-u is commonly "view source".
ctrl-I (shift i) is commonly "open developer console".
these keys are more useful to leave for the browser. ctrl-l and ctrl-u (moving
to a message without opening it) can still be had by using also pressing shift.
the previous ctrl-shift-i (show all headers) is now just ctrl-i.
this has been requested in the past on irc/matrix (i forgot who).