Kovid, thanks for the reply. I'm a novice. I understand the concept and problem, but I am not experienced enough to "see" where it is in my file. To aid my work, I run my HTML file through html & css validators at
http://www.jimworld.com/tools/. This site gives me enough information on errors to fix them and eventually be validated. I then bring the file into Calibre and save it as an epub file. At this point it looks good to me. I then run the epub file through Threepress Consulting's validator at
http://threepress.org/document/epub-validate. This is where the error message named above shows up.
Through trial and error I eliminated the unfinished element error and my file is good to go. How I accomplished it is a mystery to me. I shall continue to lumber along, refining my work and sharping my focus. Thanks for Calibre and all the hard work behind it. - Fabe