Special texts are any content on your page that has been formatted specifically to stand out from other content on a page. Special text tags include bold, italic, underline, coloured, sizing and highlighted text. Special text makes particular text stand out to the user and it is therefore more important than the other text.
Special text carries increased weighting over other content and is good for SEO
When search engines scan a page they are looking for several factors to determine what is important and what is not. Search engines look for special text in particular to help rank a page. Special text is given a higher weighting over other text because search engine believe that the words have been formatted specifically for a reason and must be important for users and thus are important for search engines too.
When should I use special text?
Special text tags should be used when it is appropriate to both users and search engines to emphasise important points. The use of special text on keywords should be encouraged though this does not mean that you should change every keyword to special text or avoid using special text on text that is not a keyword.

