This would replicate the functionality of a Microsoft component for IE ("Web Accessories for Internet Explorer"). It's the on-screen equivalent of a yellow highlighter marker. "Text Highlighter - This Web Accessory lets you highlight text in a document just like you would in Microsoft Word. It's easy. Select your text, then right- click and choose Highlight. This tool is very useful for reading long documents. " (from: http://www.microsoft.com/windows/ie/previous/webaccess/ie5wa.asp)
And what's the purpose of this? Would I be able to mark text on a site and then return the other day with the text still marked? Or is this a per-session (e.g. useless) feature?
Even if it's per-session, it's not entirely useless, especially if the highlighter effects are printable. I am confirming this because it sounds like a reasonably valid feature, especially since Microsoft offers it as an add-on.