Styles overwritten with Nextcloud 29

[cluk1]

After upgrading to Nextcloud 29 my existing Readme.md files are rendered differently than before.
H1 headings e.g. have the same font size as usual paragraphs.
Also tables column sizes seem to be messed up.

As I have upgraded from Nextcloud 27 to 29 I am not sure which version introduced this change.

Mardown to test:

# 1. Heading

| Col1 | Col2 | Col3 |
|--------------|------|----------|
| Content1 | Content2 | Content3 |