Flash was and is still great for online games. HTML5 and WebGL and other open formats still have to mature till they match the performance of a simple Flash Game but in IT terms 2020 is very far, many things can happen in 3 years.

For more complex 3D games there is Unity plugin but also major browsers seems to have a beef with this particular plugin and disable support for it one by one.

Best regards from an casual online browser-gamer ;)