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Magento allows you OOTB to merge files but not to minify.
There are several modules (free and paid) that works pretty well.
Sometimes the minified and merged files can be a little problem on backend so you can minify and merge only the files for your store front and then use the regular calls to backend.
Sorry, OOTB = Out Of The Box.
Mmm... I know there was one that works always fine (but now I can't remeber the name).
At the Marketplace there are only this alternatives: https://marketplace.magento.com/catalogsearch/result/?q=minify#q=minify&idx=m2_cloud_prod_default_pr...
But if you look thourgh GitHub there are a lot of options.
The module that I'm trying to remember, maybe was this one: https://github.com/ho-nl-fork/magento1-WBL_Minify (I'm not sure, you should check by yourself).
But again, on GitHub you'll find a lot of options. If these options fails you should take a little bit of time to try another modules or maybe you can explore paid versions.
Can you share the report? I remember the style of reports from GTMetrix but not the CSS code recomendation.
Yes, that recomendation is ok.
Also, if you use the Magento's merge feature and then some minify plugin, you'll get also en improvment on JS weight but also the number of request will be lower.
Another suggestion could be to explore HTTP/2.