Whilst speculative, hidden content presents an issue for search engines and end users – so therefore, it becomes an SEO hot potato.
Technically, black and white, hidden content is content present within a page that isn’t directly visible to the end user.
But where do you draw a line between “HIDDEN” content which is content purposely put out of the way to influence rankings vs. “hidden” content that sits inside a concertina/accordion – and does it matter?
Well, yes, yes it does. Googlebot can fetch and render pages, meaning it can detect tag nesting, content nesting inside span/js and can apply different weight to content depending on how it is hidden.
Hidden content is a grey area, because, many sites genuinely organise content in a way that allows users to choose whether they see it (concertina/according) – but, many use it for the opportunity of content stuffing.
Google can and will devalue out of sight content. If content is hidden in a way that is blatantly obvious you’ll loose the weight of that content altogether.
Nifty tip – use the cache: command in front of your URL, then select the TEXT ONLY version to see raw content – then cross evaluate that against the content you can see in the normal rendered version.