I want to make it like the picture below if a user is online. 
That’s such an incredible detailed Question wow…
My first instinct is to answer: Than show a green checkmark when a user is online 
What is the Problem about this? The Html Part or the Part about to know when your users are online?
Second one will be harder to answer as this depends on your environment, we can only give suggestions here.
the problem is not whether it is online. How can I do that green dot with css.
Thats very basic. You should learn about position: absolute which you can use to archive this: CSS Layout - The position Property
@EinfachHans Thank you. 