Answer 1

Technology is not quite there yet. Weather can not be programmed in any reasonable scale in an open environment. Or do you mean programming rainfall as in an irrigation system or maybe a small controlled environment? In that case I'd recommend using something other than plain HTML and maybe tell us what hardware you are using.

My best guess is that you want an animation like rain go across your HTML web page. If so use CSS or JS to do this as well as image(s) of rain drops. With some time I could give you code to do this, but first I want to be sure this is actually what you want.