These attributes and tags are just RoboHelps way of maintaining files, and don't appear in the output.
If I recall correctly, RoboHelp inserts the spaces tag whenever you add a couple of spaces in the WYSIWYG editor. If you don't want that, insert some non-breakable spaces (& nbsp;) in html mode. (without the space though, but if I add the html character, the forum turns it into a non-breakable space :-( )
The attributes x-right, x-cell, x-save-position, x-use-null-cells, x-next-tag, and so on help RoboHelp position elements and control how RoboHelp maintains them. Example:
in the definition of a style, makes that the next paragraph after a header is a standard paragraph with the class Body-Text. You can remove these from your style sheet, but RoboHelp may put them back whenever you edit the css trough the built-in css editor.
I wouldn't worry about the x- commands in topics, as there removed when you generate your output, and RoboHelp just adds them again when you remove them in html mode. The only way to get rid of them is to edit the html files in an external editor.
I think that only applies to RoboHelp 8. RoboHelp 7 has its own way of using process instructions, such as using a span class for conditional build tags. I don't think that is allowed in the html process instructions (http://www.w3.org/TR/html401/appendix/notes.html#h-B.3.6).
But then again, I don't really care. The output (WebHelp) is still W3C compliant when you check the option W3C compliant topics when you generate the WebHelp.
Thank you William and Peter for taking the time to answer my question. - Peter, you're right - I didn't mention I was referring to v7, but in fact I am. William must have somehow read my mind. You'll need to work on developing that skill. Or I can just be more clear when posting questions.