Unity’s HTML5 export is super heavyweight (unless you do some super complicated things getting it to work with the ECS stuff). Flash was...though abused a lot, much more lightweight, able to slot into a randomo webpage without sticking out too much/causing performance issues.

Also as a direct substitute, Unity has much inferior font rendering, and no built-in vector graphics capabilities.

If I was doing a 2D animation tool for browser export, I don’t see that unity would offer me much, and would come with a lot of downsides.