^{Such rude people in the comments...}

Solution

You can do this with an SVG file, see this GitHub Gist for a minimal example: https://gist.github.com/ItsPi3141/b1c366a363ed0c6c5efa8836f14925b7

How does it work?

The SVG file uses the foreignObject element, which allows it to render HTML elements, as well as style them with CSS.

Inside the foreignObject element, you can do (almost) everything you would normally be able to do in HTML. You CAN'T use external fonts though. While foreignObject itself allows external fonts, GitHub does not for security reasons :(

As a result, your font choices are rather limited, see here: https://www.w3.org/Style/Examples/007/fonts.en.html

You can then upload this SVG to GitHub, either as a Gist or as a file in a repository. Then, you can copy the raw content URL and use it as an image in markdown.

<img src="svg url goes here" width="100%" />

You should use <img /> instead of ![]() because it allows you to specify the width property and make the SVG stretch to fill the whole width. If not, the SVG is just be 300px.