Attempt to improve the readability of the opening paragraph

The first paragraph may have typos.

I've changed:-
1. ... are a new **browsers feature** that **provides** » ... are a new **browser feature** that **provide** ...
2. ... run **independent** of web pages ... » ... run **independently** of web pages ...
3. Unlike other Service Workers can be ... » Unlike other **workers** Service Workers can be ...
4. **not** the lack of retained references ... » **note** the lack of retained references ...

Reasoning / uncertainty:-
1. *Think* the verb "provide" should be in plural form to match "are".
2. Not sure about this one.   This *might* be my personal preference rather than being wrong so I'd be happy to put this back.
3. Again not sure - maybe this needs comma? As in "Unlike other workers, Service Workers", also maybe should be "Workers"?
4. Really, really not sure what this sentence means.  Maybe "Unlike other workers, Service Workers can be shut down at the end of events, documents do not retain references to them, and they have ...." could be clearer (assuming I'm interpreting the meaning correctly)