Well I'm certainly an ill-trained author and it seems to me that the details of the markup are wholly irrelevant to the people actually writing and making movie

Well I'm certainly an ill-trained author and it seems to me that the details of the markup are wholly irrelevant to the people actually writing and making movies. The screenwriter should be chugging along with (e.g.) a dialect of markdown that keeps the details the hell out of his way. Whether the markup that comes out the other side looks like XML or HTML just doesn't matter. And "1 page = 1 minute" is a dodgy heuristic* that could easily be supplanted by software that could process the source and know what's set direction, scene changes, dialogue, etc. * "1 page = 1 minute" has long since gone from handy heuristic to self-reinforcing delusion. Today, if a screenwriter is selling an average (hundred minutes, more or less) movie, he will ensure that his script is a hundred pages (more or less). He'll tweak margins, font sizes, spacing, descriptions and more. He doesn't want his masterpiece stigmatized when it makes the studio rounds just because it showed up thirty pages either side of proper. And so it goes.