(Note: this is more an issue with GH CSS rather than cmark-gfm or GFM)

Users are able to write HTML in markdown on GitHub. For example:

Hi, <sup><a href="#">world</a></sup>

The new footnotes use similar markup and use CSS to add brackets around footnote:

.markdown-body sup > a::before {
  content: "[";
}
.markdown-body sup > a::after {
  content: "]";
}

This results in the previous markdown being rendered as:

Hi, ^world

Would it perhaps make sense to use:

[data-footnote-ref]::before {
  content: "[";
}
[data-footnote-ref]::after {
  content: "]";
}