I am trying to embed a A.html file inside B.html(report) file as a hyperlink.
Just to be clear, both html files are offline and available only on my local machine. and at last the report html file will be sent over email.
The email recipient will not be having access to my local machine. So if they click on hyperlink in report.html, they will get "404 - File or directory not found".
Is there any way to embed A.html inside report.html, so that email recipient can open A.html from report.html on their machine
You need to one of:

Attach A.html as well as report.html,
Post A.html to a shared location such as Google drive and modify the link to point to it, or
Put the content of A.html into a hidden <div> with a show method.
Embed one html file inside other report html file as hyperlink in python