Support Speculation Rules API formatting in HTML (#12882)

* Add script formatting for `speculationrules`

* Fix lint problems

changelog_unreleased/html/12882.md

@@ -0,0 +1,32 @@
+#### Support Speculation Rules API formatting in HTML (#12882 by @sosukesuzuki)
+
+Please read https://web.dev/speculative-prerendering/ for about Speculation Rules API.
+
+<!-- prettier-ignore -->
+```html
+<!-- Input -->
+<script type="speculationrules">
+  {
+   "prerender": [
+  {"source": "list", "urls": ["https://a.test/foo"]}
+  ]
+  }
+</script>
+
+
+<!-- Prettier stable -->
+<script type="speculationrules">
+  {
+   "prerender": [
+  {"source": "list", "urls": ["https://a.test/foo"]}
+  ]
+  }
+</script>
+
+<!-- Prettier main -->
+<script type="speculationrules">
+  {
+    "prerender": [{ "source": "list", "urls": ["https://a.test/foo"] }]
+  }
+</script>
+```