Hidden text means putting words or links on a page in a way that visitors cannot see, but that crawlers can read, to influence rankings. Google's spam policies list hidden text and link abuse with examples like white text on a white background, text behind an image, text positioned off screen, zero font size, and links hidden in a single small character. Legitimate ways of collapsing content, such as tabs and accordions, are a different matter.
What is fine
Google's policy explicitly notes that not all hidden content is deceptive. Content in tabs, accordions and expandable sections that users can open is normal design, and under mobile-first indexing Google has said such content is treated as part of the page. Text for screen readers, image alt attributes and descriptive labels that improve accessibility are also fine.
How it happens by accident
A theme change can turn text the same color as a new background. A CMS might keep old SEO text in a hidden div. A hacked site may have spam links injected with display:none. A crawl that compares rendered visible text with HTML text can surface these.
Example
Leftover SEO text in a template
During a redesign audit, hypothetical CRM Mossgate found a hidden div on its homepage containing a paragraph of keyword variations, added years earlier by a previous agency. It was removed immediately, along with similar blocks on three landing pages.
Common mistakes
Avoiding accordions out of fear
Collapsible sections that users can open are fine. The issue is content users can never see.
Questions
Yes. Google has said content in tabs and accordions is indexed and treated as part of the page under mobile-first indexing.
