Update creating-and-highlighting-code-blocks.md

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

@@ -66,6 +66,11 @@ 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)
 
+> [!NOTE]
+> While both capitalized and lower-case language identifiers are allowed on GitHub, to
+> have a fenced code block highlighted on GitHub Pages sites the same way they're
+> highlighted on GitHub, you must use lower-case language identifiers.
+
 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