Hi. To answer a common use case, I think this page should mention how to define a"browserslist" config inpackage.json for Autoprefixer:
http://symfony.com/doc/current/frontend/encore/postcss.html

Maybe something like a note stating:

Autoprefixer recommends defining your target browsers inpackage.json:

{"browserslist": ["last 2 versions","ios >= 8" ]}