you have to import the Let’s Encrypt Authority X3 cert into keychain access. Here is how :
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Download the certificate file
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Search “keychain access” in spotlight and open it
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On the left of keychain access, there is a menu with your keychains. Click on the “system” keychain (under the “login” keychain)
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Locate the certificate file that you downloaded in finder, then drag and drop it into the list of items in the keychain
Now, keychain access knows who sighed it and will tell safari that.