css: bg-attachment: fixed not supported on iOS (#12463)
css: bg-attachment: fixed not supported on iOS (#12463)

* css: bg-attachment: fixed not supported on iOS

background-attachment: fixed doesn't work on iOS, as you can verify
using [this simple test](http://tools.simonsimon.nl/parallax.html)
I wrote, or see in these links:
https://bugs.webkit.org/show_bug.cgi?id=196327
https://developer.apple.com/forums/thread/99883

* Correct css/properties/background-attachment.json note

Co-authored-by: Daniel D. Beck <3d0f3b9ddcacec30c4008c5e030e6c13a478cb4f@ddbeck.com>

Co-authored-by: Daniel D. Beck <3d0f3b9ddcacec30c4008c5e030e6c13a478cb4f@ddbeck.com>
mdn/browser-compat-data,chrisdavidmills/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data