Method for updating web app data (push?)

I am building a web app (using Aurelia) that will have a chat-like component where people can comment on items. What is a good method (using current/new tech or standards) for checking for updates (new comments by other users)?

In the past I would have just polled the server every few seconds, but I'm wanting the app to be able to handle millions of users, so this would be expensive on the server side.

Have been reading about push notifications but not sure how well it's supported.