Responsive Comments board, pure HTML - CSS -JS

I guess i did a good job this time, i like it ))

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Anyway, as always i have some question, but most of all, if somebody would be so nice to check my code, i would know how to improve the JS part. Everything works exactly as i want, but i'm sure i could have done it better.

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I also added to possibility to "unchek" the notification singularly.

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I also have a doubt, i'm using the css to style the page and interacting with JS file adding and removing classes that i previously write in the css, but i often see programmers writing js directly inside the html, or in a separate file but using the .style method directly inside the js code, can you suggest me which mode you think is the best practice??

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Thank you all guys and Happy Coding :) ;)