Some questions:
how much it is important a w3c validation of a website css?
why these two w3c validators (https://jigsaw.w3.org/css-validator/validator?uri=https://www.culturanuova.net/normal.css and https://css-validator.org/validator?uri=www.culturanuova.net&profile=css3&usermedium=all&warning=1&lang=en), pointing at the same page, are so different in their judgement?
how could I fix this error Property shape-outside doesn't exist : circle(50% at 60%) margin-box ?
Thank you!
It may be how up-to-date they are with the latest CSS spec.
We would need to see the CSS concerned in full.
You don’t need to fix it. Its just that the validator hasn’t caught up yet. It’s always a long way behind on newer features so there will always be errors or warnings.
If you use the latest version of the validator then you will get less errors as you saw.
CSS level 3 + SVG
The one with all the errors is just version CSS level 3
The validator is always playing catch up so check the value on MDN to see if it’s part of the specs or not when you see an error.
If you look at MDN for say Zoom then it tells you at the top it’s non standard.
Whereas shape-outside seems to be a standard now.
Remember the validator is just a tool and not simply a sticker than you can put on your site anymore. Use it to see if you have typos or have non standard properties etc. Review it, make your decisions and then move on 
Thank you.
But which is the latest version?
https://jigsaw.w3.org/css-validator/ or https://css-validator.org/ ?
I guess the first one, because with the second one I cannot check CSS level 3 + SVG. I’m wrong?
A little thought: if it’s so, why the second has an url … more human, so to say 
(more “regular”)?
I guess that I can keep that code (shape-outside)…
I believe that the official validator is https://jigsaw.w3.org/css-validator/ and is the correct one as its hosted by the w3c and linked to in their documentation whereas https://css-validator.org/ is not the official validator even though it may have a similar codebase.
I would just stick with the official https://jigsaw.w3.org/css-validator/.
Well. Thank you!
Indeed https://css-validator.org/ is not hosted by w3c (I didn’t notice this), otherwise its url would be something like css-validator.w3.org.
Its graphical interface (and the logo as well) is quite deceptive… 