Generated by SassMeister.com.

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

// using modernizr and @root
.page-navigation {
  .branding {
    background: green;
    
    // If you do have flexbox
    html:not(.no-flexbox) & {
      background: hotpink;
    }
    // or ... (relies on modernizr [and will require JS to load before seing styles])
    html.flexbox & {
      background: hotpink;
    }
    // if you don't have flexbox (relies on modernizr [and will require JS to load before seing styles])
    html.no-flexbox & {
      background: lime;
    }
  }  
}

SassMeister-output.css

.page-navigation .branding {
  background: green;
}

html:not(.no-flexbox) .page-navigation .branding {
  background: hotpink;
}

html.flexbox .page-navigation .branding {
  background: hotpink;
}

html.no-flexbox .page-navigation .branding {
  background: lime;
}