TST: add test to io/formats/test_to_html.py to close GH6131
- [x] closes #6131
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@simonjayhawkins : Thanks for this!  Could you rebase onto `master` ? Hello @simonjayhawkins! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/tests/io/formats/test_to_html.py`](https://github.com/simonjayhawkins/pandas/blob/65d65d16ca232854caafeebf23742afbfefb8f29/pandas/tests/io/formats/test_to_html.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22588?src=pr&el=h1) Report
> Merging [#22588](https://codecov.io/gh/pandas-dev/pandas/pull/22588?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/996f361f8e6986ea1c65ccb164a4c585e1f4a027?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22588/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22588?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22588      +/-   ##
==========================================
+ Coverage   92.16%   92.17%   +0.01%     
==========================================
  Files         169      169              
  Lines       50708    50708              
==========================================
+ Hits        46734    46740       +6     
+ Misses       3974     3968       -6
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.58% <ø> (+0.01%)` | :arrow_up: |
| #single | `42.35% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22588?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/html.py](https://codecov.io/gh/pandas-dev/pandas/pull/22588/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvaHRtbC5weQ==) | `90.68% <0%> (+1.86%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22588?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22588?src=pr&el=footer). Last update [996f361...c12f547](https://codecov.io/gh/pandas-dev/pandas/pull/22588?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @gfyoung 

The rebase failed the CI checks again. I'll wait until #22597 is merged into master before trying again. @WillAyd i made a couple of additional changes to your request in the latest commit which on hindsight may not have been such a good idea.  i changed the name of the test since it no longer includes a pivot table command and secondly i took the opportunity of fixing a typo in the original problem description and so now the test does not faithfully recreate the original issue. will be happy to amend as necessary. thanks @simonjayhawkins 