media queries fail on iPhone 5

In Google Chrome emulating an iPhone5, the test page shown below works as expected: width:568 and height:320 are detected by direct media query, by Javascript media query, and by direct javascript.

On a real iPhone5, though, the direct media queries and Javascript media queries fail, even though Javascript correctly detects screen width & height. What am I missing?

<html>
<head>
  <meta name="viewport" content="width=device-width, minimum-scale=1.0, maximum-scale=1.0" />
  <style>
  @media screen and (device-height:320px)  and (device-width:568px) {
    .deviceID:after { 

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