Unless they mean Notepad++, in which case they will have syntax and, with plugins, error highlighting. Best of all, it's free!

I'd still say, if you're going to use the design view in a HTML editor, Microsoft Expression Web is the way to go. You can do proper CSS-based layouts with that, it can produce really clean markup and CSS styling, and as long as you don't set the background audio setting in Options, it's all completely valid. Shame about the price of all the software like this.