The reasons why Presto coped with less memory are complex and not really down to carefulness (there's plenty of resources put into memory consumption in both Blink and EdgeHTML), but reasons of architectural decisions that led to comparatively poor performance (JS-heavy stuff especially!) and site compatibility bugs (using different types for things that are web observable, especially), and some that led to comparative instability (heck, Chrome esp. has UI lagginess with large numbers of tabs due to IPC, but gains stability and the ability for multiple tabs to be doing stuff at once as a result).
Vivaldi is mostly formed of ex-Opera employees (esp. early, long-term ones!), and I expect they're getting comparable salaries to when they left Opera, at least (i.e., a bit below market average for the cities they're in). Why are they using JS? I can't really tell, beyond a desire to use web-technology as much as possible; I'd totally agree TypeScript or Flow would be better!
Vivaldi is mostly formed of ex-Opera employees (esp. early, long-term ones!), and I expect they're getting comparable salaries to when they left Opera, at least (i.e., a bit below market average for the cities they're in). Why are they using JS? I can't really tell, beyond a desire to use web-technology as much as possible; I'd totally agree TypeScript or Flow would be better!