Use "Clip to Evernote" - An extension for Google Chrome. With it it is possible to capture the entire page or Article, Simplified Article, etc. from website or blog.
Open in the editor of the Evernote program, where you can edit completely, as in Word.
If you are starting with an .epub file, there is an editor program in the Calibre suite which can change anything in the .epub, including the fonts:
But if you convert the file from .epub to .mobi, with any program including Calibre, it's then almost impossible to edit the resulting .mobi file. So you get stuck with ...
Try using Pandoc. I am not sure how "correct" is the HTML5...
$ pandoc file.epub -t html5 -s > file.html
Pandoc is open source. sudo apt install pandoc if not previously installed.
Without -s just the body's content is generated.
(Several options available to control html output)