CSS background image to fit width height should auto-scale in proportion

Question

I have

body {
    background: url(images/background.svg);
}

This background image's width should match the width of the page, with the height varying to keep the proportion. For instance, if the body is 600 pixels wide and the source image is 100*200 (or any other measurement), the backdrop image should end up being 1200 pixels high. If the window is resized, the height needs to adjust automatically. Can this be done?

Firefox appears to be fitting the height right now before altering the width. Maybe it's attempting to avoid cropping because height is the longest dimension. Without repeating horizontally, I want to crop vertically and then scroll.

Additionally, Chrome centers the image with no repetition even when background-repeat: repeat is explicitly specified, which is the default.