Sometimes gif animations are hard to understand because we don't know where the sequence begin.
Is there a convention to indicate the beginning (something special to add at the beginning of the sequence) ?
Or is there a way to see it by itself with some option in the browser ?
In my opinion long gifs illusrating complex actions are not very useful for that very reason.
I think it's better to create short pieces in which the beginning and end of the process are evident. Video shoud be a supporting element of what has been explained on the answer, so that users are aware of what to look for. I like short explanations, a gif, more explanation and another gif and so on. Breaking complicated processes into smaller ones.
I'd rather err on the side of oversimplifying/clarifying than making a show out of an anwser.
FYI, I have finally found that the screencast keys addon has a time option which does the job.
If you refresh a page they should start from scratch again, or perhaps open them in a new tab (Right click > View Image.)
Anyway unless you manually add a frame counter or a progress bar to your GIFs (which some software allows) there really isn't a direct way to know.
I some times bother to edit my GIFs to have have a pause or a longer frame at the beginning or end to make it clearer where it stops, but I don't always do it, and not every user bothers to do so.
