Looks like native apps for kaiOS require a special partnership (https://www.kaiostech.com/mwc-2018-kai-announces-partnerships-with-mobile-industry-giants/)
It’s possible to make a deltachat client in only html5 but it requires the use of a ebsocket to tcp proxy and without web-assembly it will be painfully slow especially on low-end devices.
So I’ll say we should contact kaiOS and ask them whether merlinux (the company that manages deltachat) could become a partner and develop a native application with a html5 frontend.