CDN method is also excellent. but I'm not like the CDN method because there are many reasons

1. You have a website with a large amount of traffic. 2. You use a shared CDN like the most common is bootstrap and other CDN. Everybody uses CDN to build projects and websites. that reason your website faces downtime by CDN.

3. Shared CND links sometimes change by the provider because they launch the new version. On your website, you use an old CDN that CDN link is broken. Your website looks ugly without CSS.

Solution

You host css and javascript files on your custom CDN. Your CDN is speedy compared to a shared CDN.

There are tons of CDN services provided right now. You use CDN provides for websites.

The second question you ask is that someone visits another website. that website use bootstrap CDN. There are not possible to share a cache with another website. When the browser does, that is a privacy issue. It is not possible. Maybe I was wrong.