this CSS gets my background to fill 100% of the screen height in iOS but there is a minor problem - when you scroll down there is initially white space, then when you release yo
You can fix this by using long transition delays to prevent the element resizing.
eg:
transition: height 250ms 600s; /*10min delay*/
The tradeoff with this is it prevents resizing of the element including on device rotating, however you could use some JS to detect orientationchange and reset the height if this was an issue.