I don't know why whatsapp have not given easy html export features, therefore we have to think outside the box for such solutions. There are different ways to approach it. I used a few of such methods to export it which I will explain below:
Whatsapp Messenger is a phone-specific app with database file stored locally in root directory, therefore export / database extraction methods are done mostly over phone and there are plenty of softwares you can use. I'll share some of those methods:
-:Export Using Whatsapp App (Official):-
-:EXPORT using 3rd Party Apps (Much more presentable in PDF/HTML formats):-
(For this method, your phone must be Android and Rooted to extract whatsapp database decryption key)
msgstore.db = messages database
wa.db = contacts database
OR
Note 1: [For Advance Users Only] You can also extract "key" file without Rooting by using following method => https://forum.xda-developers.com/showthread.php?t=2770982
Note 2: [For Advance Users Only] Most of the paid apps mentioned above are easily available on internet.