There is No Penalty For Hiding Content For Screen Readers
There is no penalty for hiding content on a page that has the sole purpose of being used for screen readers, as long as it is consistent with other content on the page. The web spam team may assess intent to deceive search engines in these cases.
Content in HTML Treated the Same Regardless of Whether it’s Visible by Default
From Google’s perspective, there is no difference if secondary content is hidden by default or included at the bottom of the page and navigated to by an anchor link, as long as the content is in the HTML.
Content Not Visible by Default Will be Shown in Search Results For Unique Queries
Content that isn’t visible by default isn’t currently being given as much weight in the search results, but it will be shown if a user is explicitly searching for that content, especially if it is unique. This content won’t be treated differently for mobile-first indexing though.
Content Not Visible By Default Will Be Taken Into Account For Mobile-first Indexing
Good user experience on mobile depends on managing content with tabs and similar methods, so content not visible by default will be taken into account for mobile-first indexing as long as it doesn’t load on click. This applies to anchor text as well.
Tabbed Content Loaded On-Click Won’t Be Indexed
Content in tabs is fine for mobile as long as it is loaded when page is loaded and not when the tab is clicked on, otherwise it won’t be indexed.
Links Not Visible By Default Will Be Crawled
Links hidden behind tabs will be crawled in the same way as other hidden content.
Content Hidden in Tabs on Mobile Will be Seen by Google After Mobile-first
Content behind tabs (or other types of user interface) on mobile will be seen by Google when Mobile-first indexing is released.
Content that Fades in Onload With ~2s Transition is Not Considered Hidden
Content is considered hidden when it is not visible at all. Content which fades in within a couple of seconds during a transition or is slightly transparent is likely to be indexed. You can test a transition using Fetch and Render.
Hidden Links are Counted
Links which aren’t visible in the rendered page should carry the same weight as visible links.