Image in Editor can render mermaid diagrams in transclusions in the editor. Which should give you an idea of the complexities WYSIWYG mode faces — and by the way, WYSIWYG mode has been moved to the “In Progress” section of the Obsidian Roadmap.
Obsidian Updates
v0.12.16 is out for Insiders!
You can now undo closing a pane.
Core now supports copying a code block in preview mode.
Empty panes will be removed on startup if the plugin that created it has been uninstalled or disabled.
{{date}} and {{time}} will now work with the core daily notes and zettelkasten prefixer plugins. Previously you needed to add the format, for example {{date:YYYY-MM-DD}}
Creating new files from in note composer will now show an error if it contains illegal characters.
You can now use Ctrl/Cmd+click or middle-click the icon or the command palette item to open daily note/new zettelkasten in a new pane