Hidden content is text content and links of a website which appear in the source code, but are invisible to the visitor. The conscious use of hidden content that gets filled with relevant keywords is considered to be manipulation of search results by search engine providers and is therefore classified as black hat SEO.
Hidden content can be used in various forms.
In all, these variations of relevant search terms do appear in the source code without being in the visible part of the website. Hidden content is therefore readable for search engines.
When search engine optimization as well as the technology of search engines was still in its beginning stages in the mid 1990s, it was relatively easy for webmasters to get their websites on the front pages of search engine results for specific keywords. A popular method of choice for this purpose was the use of hidden content. An accumulation of keywords was hidden on the website to provide the robots the necessary content for specific keywords. At that time, the search crawlers were not yet able to distinguish hidden content from actually displayed content. Today, however, search engine bots can identify hidden content very precisely.
All major search engines like Google, Bing, or Yahoo have written respective webmaster guidelines, in which hidden content is explicitly or indirectly classified as a manipulation of search results. The consequences of failure to comply with the guidelines range from a penalty for individual subpages up to the complete exclusion of a website from the index. Especially in light of the continuous optimization of search algorithms, it is therefore not recommended to integrate hidden content in your SEO strategy.
Websites are supposed to be optimized for users, not for search engines. Accordingly, as a webmaster you should ask yourself if your optimization strategies benefit the visitors of your site. In many cases, text elements can significantly alter the design of a website. Nevertheless, web designers and SEOs should work together to find a solution to accommodate text on the page despite many images. One minimal optimization option is to use the alt tags of images to describe the image with relevant keywords. Image title tags can also be used to place specific information for users as well as relevant keywords. The mass use of keywords or keyword combinations is to be avoided though, as this could be interpreted in the extreme case as keyword stuffing by the search engine bots.
In order to ensure an improved appearance of a website, you can utilize mouseover text and buttons to show and hide text. It is always recommendable to place as much visible text as possible “above the fold”.