frontmatters are not managed as separate text blocks (here: ``tags: todo`)

Can you please explain more here? I don't see any issue here

tables are not aligned (the table here was aligned in the Markdown document). The reason is that the font you're using is not a monospace font

Mono font is normally used when writing code. Even if mono font is used, table columns still can not be aligned when number of characters are different between rows (which is majority of the case). I checked GitHub and it behaves the same.

Well, that is not entirely true. Here is how Gitea displays a table in a Markdown document:

As per the frontmatter, it shgould IMO be displayed in a different color/font. Here is how Obsidian does it:

1. Editing

2. Preview

As it is currently displayed in OneDev, frontmatters appear like a head title, which they are not.

Uses mono space will not help table align as you see the second column still can not be aligned due to different number of characters. Also using monospace font for markdown will make it less readable as monospace is not the best font for reading...

As to front matters, OneDev currently does not have that concept. Heading is just heading with large size in OneDev.