The alternative was to have multiple scripts or stylesheet links on one page, which would trigger multiple HTTP requests. Multiple requests mean multiple connection handshakes for each link “hey, I want some data”, “okay, I have the data”, “alright I heard that you have the data, give it to me” (SYN, ACK, SYNACK). Even once the connection is created there is a feature of TCP called TCP slow start that will throttle the speed of the data being sent at the beginning of a request to a slower speed than the end of the request. All of this means transferring one large request is faster than transferring the same data split up into several smaller requests.

many CDNs nowadays ultimately source from npm (before caching the JavaScript on their own edge servers). So you'll be guaranteed to have the same library version on the CDN as on npm

While stylesheets can be reworked relatively easily with AMP by inlining the CSS, the same is not true for JavaScript. The tag 'script' is disallowed except in specific forms. In general, scripts in AMP are only allowed if they follow two major requirements: All JavaScript must be asynchronous (i.e., include the async attribute in the script tag). The JavaScript is for the AMP library and for any AMP components on the page. This effectively rules out the use of all user-generated/third-party JavaScript in AMP except as noted below.

The problem is that this is an external stylesheet reference. In AMP, to keep the load times of documents as fast as possible, you cannot include external stylesheets.