To html encoding add
- [x ] closes #xxxx
- [x ] tests added / passed
- [x ] passes `black pandas`
- [x ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x ] whatsnew entry

Hello @ailurus1991! Thanks for opening this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:

* In the file [`pandas/core/frame.py`](https://github.com/pandas-dev/pandas/blob/12fb0c5c5191d2141cbc529ca2d5a0382de048b8/pandas/core/frame.py):

> [Line 2258:89](https://github.com/pandas-dev/pandas/blob/12fb0c5c5191d2141cbc529ca2d5a0382de048b8/pandas/core/frame.py#L2258): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (89 > 88 characters)



