CSS背景图片不会缩放

I'm trying to learn html and css. Right now I'm trying to make a background image fill the browser window no matter the size. I've googled it 100 times and every page pretty much says the same thing. So I copied the code and it isn't working for me. When I set my browser to a smaller size, the background image repeats itself. Which I don't want I just want it to fill the window. Here's my css.I'm trying to learn html and css. Right now I'm