This note preserves the original context of the Facebook post. Some tools, interfaces, and external links may have changed.
This post contains a working example related to mobile-first content indexing.
The website in the case uses Dynamic Serving, and the server will return different versions of HTML based on the User-Agent, but the URL remains the same.
This approach requires attention to whether the cache server knows that the response content will change with the User-Agent, so the case specifically checks the Vary: User-Agent HTTP Response Header.
If the cache only saves a copy of the HTML, it may mistakenly serve the desktop version to mobile users, or the mobile version to desktop users.
This content was published in 2020, and the policy background of mobile indexing may be different from today’s implementation environment.
The long-term caveat is that whenever the same URL returns different content based on request conditions, caching, crawling, and user experience must be examined together.
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