Web developers will need to be aware of updated guidelines from Google. The recent update of Google’s guidelines for mobile-first indexing includes new guidelines for several areas. Having a website subject to mobile indexing on Google will not be optional, as the plan is for all websites to be subject to the indexing. 

New guidelines were released for:

• Crawling: If you have both mobile and desktop versions of your site, the new guidelines call for using the same meta robots tags for both versions. Because Googlebot won't load content that requires some type of user action - swiping, scrolling or clicking - to finish loading the page, lazy loading is not recommended. If your desktop and mobile resources have different URLs, be sure they’re not blocked by a disallow directive. Otherwise, Google won’t crawl those resources.
• Mobile and desktop content should be the same: Google has the following warning now added to this guideline: “If it’s your intention that the mobile page should have less content than the desktop page, you can expect some traffic loss when your site is enabled mobile-first indexing, since Google can’t get as much information from your page as before.”
• Identical structured data: Both versions of your site should have identical structured data. In addition, structured data for your mobile site should use mobile URLs.
• Visual content: This new section guides image and video content. Images should use persistent URLs and alt text should be the same for desktop and mobile sites. New warning from Google: If the image URLS are different on your desktop and mobile sites, you may see a temporary drop-off in image traffic during the mobile-first indexing transition. This also applies to videos.
• New “additional best practices for separate URLs” recommendations:
  • Error page statuses should be identical on desktop and mobile sites. Otherwise, the page will be missing from the index if a page on one site shows an error status but the same page on the other site does not.
  • Fragment URLs should not be used on your mobile site, as they’re not indexable, so they will be missing from the index after your site is transitioned to mobile-first indexing.
  • Desktop versions with different content from the mobile version will need to be made equivalent to the mobile version, to prevent desktop pages with different URLs from disappearing from the index.