REGR: <th> tags for notebook display closes #28204
- [ ] closes #28204
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

not added whatsnew. target 0.25.2? Was it a regression in 0.25 or earlier? haven't checked thoroughly but i think the bug was introduced in  0.25.1 #27991 With 0.25.0

```python
In [2]: df = pd.DataFrame({"a": 1}, index=pd.MultiIndex.from_product([['a'], [2, 3]]))

In [3]: print(df._repr_html_())
```

```html
<div>
<style scoped>
    .dataframe tbody tr th:only-of-type {
        vertical-align: middle;
    }

    .dataframe tbody tr th {
        vertical-align: top;
    }

    .dataframe thead th {
        text-align: right;
    }
</style>
<table border="1" class="dataframe">
  <thead>
    <tr style="text-align: right;">
      <th></th>
      <th></th>
      <th>a</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <th rowspan="2" valign="top">a</th>
      <th>2</th>
      <td>1</td>
    </tr>
    <tr>
      <th>3</th>
      <td>1</td>
    </tr>
  </tbody>
</table>
</div>
```

and with 0.25.1

```html
In [3]: print(df._repr_html_())
<div>
<style scoped>
    .dataframe tbody tr th:only-of-type {
        vertical-align: middle;
    }

    .dataframe tbody tr th {
        vertical-align: top;
    }

    .dataframe thead th {
        text-align: right;
    }
</style>
<table border="1" class="dataframe">
  <thead>
    <tr style="text-align: right;">
      <th></th>
      <th></th>
      <th>a</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td rowspan="2" valign="top">a</td>
      <td>2</td>
      <td>1</td>
    </tr>
    <tr>
      <td>3</td>
      <td>1</td>
    </tr>
  </tbody>
</table>
</div>
```

So it seems to have been a 0.25.1 regression (so target for 0.25.2) thanks @TomAugspurger . will do.