TST: catch warnings in test_css
closes #16033

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16094?src=pr&el=h1) Report
> Merging [#16094](https://codecov.io/gh/pandas-dev/pandas/pull/16094?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f0bd908336a260cafa9d83c8244dd1a0a056f72d?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16094/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/16094?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16094   +/-   ##
=======================================
  Coverage   90.84%   90.84%           
=======================================
  Files         159      159           
  Lines       50776    50776           
=======================================
  Hits        46128    46128           
  Misses       4648     4648
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.62% <ø> (ø)` | :arrow_up: |
| #single | `40.35% <ø> (ø)` | :arrow_up: |


------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16094?src=pr&el=footer). Last update [f0bd908...b01c4b7](https://codecov.io/gh/pandas-dev/pandas/pull/16094?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
