Invisible On-Page factors are, as you correctly guessed, parts of a web page that are not visible to the human readers. They can be read howeber, by a search engine parsing a web site. invisible page factors include:
Meta Description
Meta Keywords
Alt and title attributes
Page structure considerations
Post by : Oka Dayendra
Post on : 01 July 2011
Title : Invisible On-Page Factors
Category : SEO Guide
Source : http://www.leakbali.com