Document the fact that Rouge only recognises lower-case language identifiers (#35697)

Co-authored-by: Felicity Chapman <[email protected]>

Author: nisbet-hubbard

content/get-started/writing-on-github/working-with-advanced-formatting/creating-and-highlighting-code-blocks.md

@@ -66,6 +66,9 @@ This will display the code block with syntax highlighting:
 
 ![Screenshot of three lines of Ruby code as displayed on {% data variables.product.prodname_dotcom %}. Elements of the code display in purple, blue, and red type for scannability.](/assets/images/help/writing/code-block-syntax-highlighting-rendered.png)
 
+> [!TIP]
+> When you create a fenced code block that you also want to have syntax highlighting on a GitHub Pages site, use lower-case language identifiers. For more information, see [AUTOTITLE](/pages/setting-up-a-github-pages-site-with-jekyll/about-github-pages-and-jekyll#syntax-highlighting).
+
 We use [Linguist](https://github.com/github-linguist/linguist) to perform language detection and to select [third-party grammars](https://github.com/github-linguist/linguist/blob/main/vendor/README.md) for syntax highlighting. You can find out which keywords are valid in [the languages YAML file](https://github.com/github-linguist/linguist/blob/main/lib/linguist/languages.yml).
 
 ## Creating diagrams

content/pages/setting-up-a-github-pages-site-with-jekyll/about-github-pages-and-jekyll.md

@@ -110,6 +110,9 @@ To make your site easier to read, code snippets are highlighted on {% data varia
 
 By default, code blocks on your site will be highlighted by Jekyll. Jekyll uses the [Rouge](https://github.com/rouge-ruby/rouge) highlighter (which is compatible with [Pygments](https://pygments.org/)). If you specify Pygments in your `_config.yml` file, Rouge will be used as the fallback instead. Jekyll cannot use any other syntax highlighter, and you'll get a page build warning if you specify another syntax highlighter in your `_config.yml` file. For more information, see [AUTOTITLE](/pages/setting-up-a-github-pages-site-with-jekyll/about-jekyll-build-errors-for-github-pages-sites).
 
+> [!NOTE]
+> Rouge only recognizes lower-case language identifiers for fenced code blocks. For a list of supported languages, see [Languages](https://rouge-ruby.github.io/docs/file.Languages.html).
+
 If you want to use another highlighter, such as [highlight.js](https://github.com/highlightjs/highlight.js), you must disable Jekyll's syntax highlighting by updating your project's `_config.yml` file.
 
 ```yaml