You can use it fully.
When you configure your cache so that the browser checks whether the file has been modified or not, it will request the file, and the CDN will respond with the status code 304, which means that the file in the browser cache can be used. However, this still requires an HTTP request. You can also use etag or expiration so that it does not.
However, it may happen that you download external files from sites where you do not have control over the header. In this case, in order not to have an HTTP request and a 304 response, you could save time by using local storage.
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