Anoying system pop-ups

Hi,,
When I surf a little bit, I sometimes get these anoying pop-ups (No, not new windows) but system-pop-ups saying: to view this site correctly it is necassary to download this......

My question is what is THIS a font-set or is it because of the language set in the doc-type of the document?

The reason I want to know this is very simple, I hate it and do not want viewers of my site will have the same.

Thanks,
Jerome

Well it can depend what exactly it is telling you to download. Sometimes, if for example, the page encoding is a different language that yours even though the text is in your language the browser will want to download the characterset for it.