You can install a browser eextension (i think name is kiwix only) which can load offline zim files. problem is that ux is very bad (you need to load a zim file each time, or manually change). on desktop, my answer was to manually unpack all zim files (using zimutils) and then arrange them in a controlled dir structure, then recompress them into a mountable file format, and separately, maintain a list of all files in side, and while using, i have something hand rolled to mount the archive, select suitable file, and open in browser - yes it is a lot of work, but i do kinda have a offline search engine now.