Adds dataLanguage property to the replacement <pre> element.

.changeset/cold-snakes-train.md

@@ -0,0 +1,42 @@
+---
+"@astrojs/markdown-remark": minor
+---
+
+Adds a `data-language` attribute on the rendered `pre` elements to expose the highlighted syntax language.
+
+For example, the following markdown fenced-codeblock will expose `data-language="python"`:
+```
+\```python
+def func():
+    print('Hello Astro!')
+\```
+```
+
+This allows retrieving the language in a rehype plugin:
+```js
+// myRehypePre.js
+import { visit } from "unist-util-visit";
+export default function myRehypePre() {
+  return (tree) => {
+    visit(tree, { tagName: "pre" }, (node) => {
+      const lang = node.properties.dataLanguage;
+      [...]
+    });
+  };
+}
+```
+
+Note: if using the built-in `<Code>` component, the output of the component being flattened html, the replacement `<pre>` cannot be accessed using `{ tagName: "pre" }`.
+
+
+It also allows to use the `data-language` attribute in css rules:
+```css
+pre::before {
+    content: attr(data-language);
+}
+
+pre[data-language="javascript"] {
+  font-size: 2rem;
+}
+```
+

packages/markdown/remark/src/rehype-prism.ts

@@ -9,7 +9,7 @@ export const rehypePrism: Plugin<[], Root> = () => {
 			let { html, classLanguage } = runHighlighterWithAstro(language, code);
 
 			return Promise.resolve(
-				`<pre class="${classLanguage}"><code is:raw class="${classLanguage}">${html}</code></pre>`
+				`<pre data-language="${language}" class="${classLanguage}"><code is:raw class="${classLanguage}">${html}</code></pre>`
 			);
 		});
 	};

packages/markdown/remark/src/shiki.ts

@@ -102,7 +102,10 @@ export async function createShikiHighlighter({
 
 							// Replace "shiki" class naming with "astro-code"
 							node.properties.class = classValue.replace(/shiki/g, 'astro-code');
-
+
+							// Add data-language attribute
+							node.properties.dataLanguage = lang;
+
 							// Handle code wrapping
 							// if wrap=null, do nothing.
 							if (wrap === false) {