HTML

From https://help.obsidian.md/Editing+and+formatting/Obsidian+Flavored+Markdown:

Obsidian supports CommonMark, GitHub Flavored Markdown, and LaTeX. Obsidian does not support using Markdown formatting or blank lines inside of HTML tags.

Emphasis by me. Now, let's see...

<div>
  <p>
A. Plain
**Bold**
_Italic_

B. Plain ==Highlight== [[Roadmap]]
  </p>
  <p>C. Plain **Bold**</p>
</div>

<span>D. Plain **Bold** ==Highlight==</span>

D. Plain Bold Highlight

Obsidian renders every inner text as un-styled plain text in editing view. However, in reading view, it renders A and C as plain text but B and D as normal styled Markdown content.

They seems to use customized parser for the editing view and existing CommonMark (or GFM) parser for the reading view.