XDG_UTILS_DEBUG_LEVEL=2 xdg-mime query default text/html is helpful to get insight into where xdg-mime looks to determine how to open these files.
Based on that I found Changing programs used to launch an Application in Gnome3 - #2 by jtojnar, which I can confirm works. You only need the environment.etc addition, the environment variable is OK by default nowadays (at least on unstable).