The situation when the browser sent a request and has a pending connection with the server, is standard for this method. Only when a message is delivered, the connection is reestablished.

This is a horrible sentence in general. As a newbie, I have to read this 5 times, I don't even know what the main point is here. Is it just 'This is a standard method in most cases'? The rest of the sentence is repeating what the article already says:

1. A request is sent to the server.
2. The server doesn’t close the connection until it has a message to send.
3. When a message appears – the server responds to the request with it.
4. The browser makes a new request immediately.

Also, I am confused by this:

Only when a message is delivered, the connection is reestablished.

This is conflicting with this:

2. The server doesn’t close the connection until it has a message to send.

How can connection be reestablished? It was never closed. Maybe the author was meant to say 'Only when a message is delivered, a new request is made'.

If I get time, I am trying to review all the content for these problems #3335.

A small recommendation, change the title of this PR to something more descriptive. For example: 'Check grammar/legibility for 'long polling' article' instead of 'grammar & legibility (to check)'.

Well, @p2635 , I tried to fix both issues here.
Any review/advice from a native Eng speaker would be nice...

I am a native English speaker, but I still need the original author to answer my questions above. I'm not a maintainer of the project, so I don't get to make the final decisions. We just need to be patient.