[Bug] `~~~` with any syntax highlight designator except `txt` does not apply highlighting, and renders with a `font-size` < `1em`.

[RokeJulianLockhart]

As originally discussed at github-linguist/linguist/issues/7637, when a CommonMark <pre><code> (~~~) with a syntax highlight declaration (~~~RegEx) is utilised inside a Markdown footnote, its font-size reduces, and the syntax highlight designation is not applied, such that it reproduces issues/1851:

Reproductive Markup

[^1]

[^1]: ~~~MA
    "(?:(?:headers)|(?:cookies))"\s*:\s*(?:\{[^{}]*\}|\[[^\[\]]*\]|"[^"]*"|[^,}\]]*)(?:,)?\s*
    
    ~~~

Comparison Of Correct And Incorrect Presentation

~~~RegEx	~~~txt