For what little it may be worth, I looked at the lxml_html_clean project on GitHub. I downloaded the project.zip file and copied the lxml_html_clean directory from the zip file into the C:\Users\<user>\AppData\Local\sigil-ebook\sigil\plugins\HTML2Epub directory. I then edited the cutils.py file and edited line 15.
Code:
Original:
import lxml.html.clean as clean
Code:
Replacement:
import lxml_html_clean.clean as clean
At that point HTML2ePub came up and I imported an HTML into an ePub.
After some testing, I found that I preferred using Sigil's builtin add existing file tool for adding HTML to an ePub. HTML2ePub seems to require that I have either a <h1> or <h2> header and requires title, author, publisher, etc. be entered.
I've attached the modified plugin to this message.