2
rg_software
The fit-to-screen toggle works marvelously. I also checked the preset flag FullScreen=0/1; both settings work as expected. Great job.

I also have two MHT files that fail to display properly. One gives an empty screen, the other is displayed in raw text mode only, regardless of the setting of the flag DetectEncoding=0/1. MarkdownViewer displays them both fine.
I have another multi-page MHT in which EdgeViewer does not show the top menu line properly, but as floating unsorted items in orange font. MarkdownViewer displays the top line correctly in white font on a blue background. EdgeViewer is more capable of properly showing the pages in correct formatting, whereas MarkdownViewer loses the formatting on page two and onwards.
I bundled the three mentioned MHT files in an 7zip archive (~4.4 MB):
Download page
Enjoy yourself, everyone.
EDIT: fullscreen toggle > fit-to-screen toggle