Custom css file for readme.md in a Github repo

Question

GitHub does not allow for CSS to affect README.md files through CSS for security reasons (as if you could inject CSS into a ReadMe, you could easily launch a phishing attack). This includes both stylesheets referenced through <link rel> and inline styles used with <style>.

The readmes are in markdown syntax, so some styling can be done, such as adding colours through placeholder images, just like here on StackOverflow. For example, you can add red squares with the following:

- ![#f03c15](https://placehold.it/15/f03c15/000000?text=+) `#f03c15`

You can also make use of things like diff, json, html, js and css to affect text colouring.

You can add some HTML (actually XHTML) and CSS inside a <foreignObject> tag inside of an svg file and then embed that inside of an <img> tag in your GitHub README.

This is a simple animation in CSS that changes the color of the h1 text:

h1 {
  color: red;
  animation: myanimation 2s infinite;
}

@keyframes myanimation {
  from {
    color: red;
  }
  to {
    color: yellow;
  }
}

<h1>Hello world!</h1>

You can embed the style and HTML into a <foreignObject> tag inside of an svg like so:

example.svg

<svg fill="none" viewBox="0 0 400 400" width="400" height="400" xmlns="http://www.w3.org/2000/svg">
    <foreignObject width="100%" height="100%">
        <div xmlns="http://www.w3.org/1999/xhtml">
            <style>
            h1 {
                color: red;
                animation: mymove 2s infinite;
            }

            @keyframes mymove {
                from {
                    color: red;
                }
                to {
                    color: yellow;
                }
            }
            </style>
            <h1>HELLO WORLD!</h1>
        </div>
    </foreignObject>
</svg>

Then, lastly you can embed the svg in your README, using an <img> tag and it should render your HTML with the applied CSS styles:

README.md

# My GitHub README

Welcome to my README!

<div align="center">
    <img src="example.svg" width="400" height="400" alt="css-in-readme">
</div>

another example & source