Add HTML

Not really. You are thinking straight HTML because that is what you are dealing with. But a browser can do lots lots more - JavaScript, CSS, ... If their Run modes tried to emulate this, they would be forever. That is why they provided the Preview in Browser button. Further, not all browsers are the same - so they would have a ton of work. Much easier to provide the Preview button. Not really. You are thinking straight HTML because that is what you are dealing with. But a browser can do lots lots more - JavaScript, CSS, ... If their Run modes tried to emulate this, they would be forever. That is why they provided the Preview in Browser button. Further, not all browsers are the same - so they would have a ton of work. Much easier to provide the Preview button.