WebHelper is not exactly a preview for your currently edited HTML file. Most web development nowadays are a lot more than one HTML file, the page delivered to the client is usually generated a way or another; so it is seldom enough to simply display the page you're editing. Because of this, the WebHelper plugin is displaying a URI of your choice (here in your example, it's displaying `about:blank`), which can be any scheme supported by WebKit: `http`, `file`, etc.
It was suggested to add a feature to ease displaying the currently edited file, but there was not enough interest for anybody to implement it. If you wanted real-time update (instead of having to save the file first), one could investigate the idea of loading a data URI that changes dynamically, or any other available mean through the WebKit API.
Anyway, basically to see you page you need to make the WebHelper URI point to `file:///home/caltrop/Web Page/LINUX.htm`.