How do I view the source of Markdown files on Github?

If you don’t want to use the “raw” button, you can (since June 2021) add ?plain=1 to your GitHub markdown file URL:

Appending ?plain=1 to the url for any Markdown file will now display the file without rendering.

As with other code files, it will also show line numbers, and can be used to link other users to a specific line or lines.
For example, appending ?plain=1#L52 will highlight line 52 of a plain text Markdown file.

Example:

https://github.com/git/git/blob/master/README.md?plain=1#L49-L51

Since Sept. 2021, there is a button which adds the ?plain=1 for you: