I'll be using the text editor to clean up the bloat from a Word doc saved as "web page filter" (ie. html). Then I am going to convert the .html into a .epub. Eventually it will be on Amazon et al. as an ebook (novel).
I am using Xubnutu 13.10, 32 bit.
What I have used that worked very well for me in the past (and I know this is an epub editor) was Sigil. When I'm working there and I make an error in an html tag right away a big red warning pops up in the companion "view" screen and then I stop and set right the error.
The key feature I'm looking for is this code violation notification. Other features would be snytax highlighting and search and replace. And it would be nice if it was available for free.