<div> content is ignored with KaTeX lab

### Description

```
{
"type": "m.room.message",
"content": {
"msgtype": "m.text",
"format": "org.matrix.custom.html",
"body": "

without div",
"formatted_body": " without div"
}
}
```


### Steps to reproduce

- `/html` or any other means of creating above event

Element can ignore the structure given by the tag, and it's other properties, but should at least show the content?
See also parity with android, which does show it as expected.

Possibly Element could warn about tags not being supported when sending `/html`.

Logs being sent: no

### Version information

- **Platform**: desktop

For the desktop app:

- **OS**: Arch
- **Version**: aur/element-desktop-nightly-bin 2021061001-1