As best I can tell, success = luck * skill.

The big successes that I've seen all have exceptional founders, but that's clearly not enough. Timing is a huge component as well. Often a good idea will fail simply because the market or technology isn't ready. My own experience with Gmail reflects some of that. When we decided to write the whole frontend in JS, everyone said that it was a bad idea doomed to failure. It had been tried before (e.g. desktop.com) and had always been a disaster. They were right about the past, but wrong about the future. We released Gmail right around the time that browsers were finally getting good enough, and were were very careful to keep our code as fast and slim as possible.

"As best I can tell, success = luck * skill."

The actual formula that academics use is Performance = AMO, ability * motivation * opportunity. I think there are a couple of modified versions but I can't remember what they are off the top of my head.

I've found that in instances where my ability is 7 and motivation is 34 but opportunity is something like, say, 23, I've had better performance than when my ability is 22 and my motivation is 12 compared with a 13 opportunity.

This is to say that my experience confirms that there probably is some completely subjective formula that will fool people into thinking there is some reason behind my good luck. :-)

I am sure that you knew that browsers were finally getting good enough. Getting the timing right is still difficult but it is not pure luck. I suppose you also need some luck (and skill) to convince others that something is now becoming possible. Finding an investor that also sees the opportunity can be hard.