For the simple case of electron-positron annihilation, they transform into high-energy photons, whose total energy is equal to the total mass-energy of the electron and positron. See: https://en.wikipedia.org/wiki/Electron–positron_annihilation

This means that when developing your application you need to keep track of what differences each browser has, and write/use polyfills or conditionals to ensure that your methods work as expected on all platforms.

core-js has existed for nearly a decade

that puts a barrier on new users and enforces echo chambers

Only if trust starts at 0. A system where trust started high enough to not filter out posts and comments would avoid that issue.