The way we tell Google about our separate urls requires some work.
When you have two different urls, one for mobile, and one for desktop Google needs to know when each page is appropriate to use.
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The way we tell Google is called the Vary HTTP header. The Vary HTTP header tells Google that the content it is crawling may look different depending on who is looking at it.
By declaring the Vary: User-Agent header you are telling Google your pages have a mobile alternative.
By declaring a viewport, your web page can render correctly on any device. When a site is using dynamic serving it is providing different content to desktops users than it is to mobile users.
Google can not detect automatically when content is being dynamically served, so we have to tell Google specifically what is happening.
Make sure mobile redirects are not hurting your page speed.