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For this very reason I am trying to buy old mobile phone so that I could test my web app on it. At present I am looking for either older versions of Android phones(very difficult to find) or use phones with Android GO or KaiOS. But both Android GO and KaiOS have "modern web browser" so that is the dilemma I am having while choosing a mobile phone.

For the laptop/desktop, I try to stick to older time-tested front-end libraries such as jQuery and Bootstrap.

I read an article recently where a developer mentioned that he was very easily able to "port" his desktop web app to low-cost small form-factor mobile devices because he is using Svelte. I have not used it, but if developers want to target low-cost devices(though with modern browsers) then they could try Svelte. But to support old browsers/devices I think sticking to simple old time-tested libraries helps.