This is an accessibility problem. Some buttons are invisible when using the high contrast theme. See GNOME #785289.
For instance, the text for the Submit button on this Bugzilla is not visible. Nice that WebKit re-renders everything when the theme is changed, so you can scroll down and test without even reloading the page.
https://bugzilla.gnome.org/show_bug.cgi?id=785289 for ease of click.
Created attachment 316226 [details] Screenshot Screenshot of the buttons. The bottom-left button is currently pressed.
Created attachment 316227 [details] Screenshot with Adwaita Same screenshot with Adwaita, for reference
WebKit doesn't use foreign drawing for this anymore + the HC theme isn't as extreme anymore. Closing.