I'm having trouble finding documentation for this. Is this Safari specific?
There was a recent bug in iOS 9 http://kihlstrom.com/2015/shrink-to-fit-no-fixes-zoom-problem-in-ios-9/, the solution to which is adding the
It is Safari specific, at least at time of writing, being introduced in Safari 9.0. From the "What's new in Safari?" documentation for Safari 9.0:
Viewport meta tags using
"width=device-width"cause the page to scale down to fit content that overflows the viewport bounds. You can override this behavior by adding
"shrink-to-fit=no"to your meta tag as shown below. The added value will prevent the page from scaling to fit the viewport.
<meta name="viewport" content="width=device-width, initial-scale=1.0, shrink-to-fit=no">
In short, adding this to the viewport meta tag restores pre-Safari 9.0 behaviour.