To all who took the time to explain this to me, a special thanks for sharing your expertise. I used your suggestions to start again.

To recap the old workflow: doc>htm>Calibre>epub produced a file that miserably failed Epubcheck and would not deliver to iTunes.

Rather than use Sigil to validate and correct that file, I tried doc>htm>Sigil>epub.

An Epubcheck of this file produced only one error 38 times, corresponding to chapter breaks:

"ERROR: /Volumes/dummy/OEBPS/Text/Section0001.xhtml(645): attribute "clear" not allowed at this point; ignored"

My question is now reduced to how I correct this single item in Sigil.

If you help me do that, I promise to quit being a pest . . .

You probably have something like:

<br clear="both"/>

for pagebreaks. Don't use that. Instead, insert chapter breaks and split the file at them.

Thanks, Jellby:

The doc file has Word page breaks. When I first opened it in Sigil, the text continually flowed, so I inserted chapter breaks to split the file. Since the original Word page breaks are the problem, would it be better for a newbie to fix that within Sigil, or to remove all the page breaks from the original doc file, re-open it in Sigil as a new book, and insert the chapter breaks?

Thanks for your advice.

How did you get the file in Sigil? If you made an export from Word to filtered HTML, the line breaks are converted correctly.