You will need to install a user script manager extension to install this script.
I do not personally use every single supported site. Please let me know if there are problems with script functioning or legitimacy of any particular one.
The popup window size and other parameters were chosen to fit the activation page's desktop layout on a 1080p screen without scrollbars. If you want a more compact mobile layout, adjust the parameters for yourself as you see fit. I chose not to because there are issues on Steam's end where any error messages in product activation are scrolled off-screen. This (and other issues) could possibly be handled in the userscript but for the time being I've elected not to touch pages originating from Steam. Thanks to @psydex for confirming the v1 series script broke because of a Steam change (and not just for me) and prompting the release of v2.