No. In the strict sense you can't. HTML was mainly invented to design and exchange documents written for a browser (and CSS provides layout information for such a document).

Although, that is not the case anymore. There are many possibilities, where HTML and CSS is used outside of the browser (e.g Gnome Shell is developed with the help of CSS and Javascript or think of Modern UI apps for Windows 8).

So if you consider Windows 8, there is the possibility of creating an app outside of the browser; but technically it is not an .exe It is not an executable.

Another possibility is to make use of the WebBrowser Class, which allows you to write an .exe which makes use of your HTML and CSS.

So there are ways to stuff HTML into an executable, but you can not make it executable by itself, since it is a document/markup language and not a programming language and can not be converted to running code.