We actually spent some time making sure that we weren't going to run into problems with browsers. However, as the OP points out, because LE had a cross-signature from an existing CA, browsers didn't have to any positive action to make LE certificates work. This was absolutely essential to getting things off the ground.
Oh, I know you all did and I remember the cross-signing. I worried that you'd get slapped down somehow, that the crappy cert companies would find a way to stop/reverse it, that the project would fizzle out, etc. I thought it was cool as hell but it seemed something so clearly good couldn't stay good but you all have only gotten better over time.
