What is Injected stylesheets in Chrome?

Question

css

What are Injected stylesheets and where do they come from?
When laying out, I noticed that in chrome there are certain styles that cannot be removed even in dev tools.
For example:

injected stylesheet 
...
h2 { 
 font-style: italic; 
}

It is treated only with the help of !important

Answer 1

I got this addblock in chrome. disabled it on the domain and it worked

Answer 2

I had the "Hippo Video" Chrome Opera extension installed, after disabling it, the extra styles were removed, but how to deal with it so that these styles are not included from other extensions?

Answer 3

And my JSON Discovery inserted an extra indent from above