Fix HTML attribute name normalization (#19709)

tests/format/html/attributes/lowercase/__snapshots__/format.test.js.snap

@@ -0,0 +1,38 @@
+// Jest Snapshot v1, https://jestjs.io/docs/snapshot-testing
+
+exports[`snippet: #0 format 1`] = `
+====================================options=====================================
+parsers: ["html"]
+                                                      printWidth: 80 (default) |
+=====================================input======================================
+<div
+  CLASS="should print as lowercase"
+  UNKNOWN="should print as UPPERCASE"
+>
+  Tag with attributes
+</div>
+<span
+  CLASS="should print as lowercase"
+  UNKNOWN="should print as UPPERCASE"
+>
+  Tag without attributes
+</span>
+<unknown
+  CLASS="should print as UPPERCASE"
+  UNKNOWN="should print as UPPERCASE"
+>
+  Non-html element
+</unknown>
+=====================================output=====================================
+<div class="should print as lowercase" UNKNOWN="should print as UPPERCASE">
+  Tag with attributes
+</div>
+<span class="should print as lowercase" UNKNOWN="should print as UPPERCASE">
+  Tag without attributes
+</span>
+<unknown CLASS="should print as UPPERCASE" UNKNOWN="should print as UPPERCASE">
+  Non-html element
+</unknown>
+
+================================================================================
+`;

tests/format/html/attributes/lowercase/format.test.js

@@ -0,0 +1,39 @@
+import { elementAttributes } from "@prettier/html-attributes";
+import { htmlTags } from "@prettier/html-tags";
+import { outdent } from "outdent";
+
+const tagWithAttribute = elementAttributes.div
+  ? "div"
+  : htmlTags.find((tag) => elementAttributes[tag]);
+const tagWithoutAttribute = !elementAttributes.span
+  ? "span"
+  : htmlTags.find((tag) => !elementAttributes[tag]);
+
+runFormatTest(
+  {
+    importMeta: import.meta,
+    snippets: [
+      outdent`
+        <${tagWithAttribute}
+          CLASS="should print as lowercase"
+          UNKNOWN="should print as UPPERCASE"
+        >
+          Tag with attributes
+        </${tagWithAttribute}>
+        <${tagWithoutAttribute}
+          CLASS="should print as lowercase"
+          UNKNOWN="should print as UPPERCASE"
+        >
+          Tag without attributes
+        </${tagWithoutAttribute}>
+        <unknown
+          CLASS="should print as UPPERCASE"
+          UNKNOWN="should print as UPPERCASE"
+        >
+          Non-html element
+        </unknown>
+      `,
+    ],
+  },
+  ["html"],
+);