@@ -0,0 +1,13 @@
+import { describe, expect, it } from "vite-plus/test";
+
+import { resolveHighlightLanguage } from "./highlightLanguage";
+
+describe("resolveHighlightLanguage", () => {
+  it.each([
+    ["src/init.luau", "lua"],
+    ["styles/page.xsl", "xml"],
+    ["styles/page.xslt", "xml"],
+  ])("%s highlights as %s", (path, language) => {
+    expect(resolveHighlightLanguage({ path })).toBe(language);
+  });
+});
@@ -1,8 +1,16 @@
 import "@t3tools/client-runtime/assembly-highlighting";
-import { getFiletypeFromFileName } from "@pierre/diffs/utils/getFiletypeFromFileName";
+import {
+  getFiletypeFromFileName,
+  setCustomExtension,
+} from "@pierre/diffs/utils/getFiletypeFromFileName";
 
 import { HIGHLIGHTS_LANGUAGE_IDS } from "./highlights.generated";
 
+// Highlights has no lexer for these dialects, so they use their base language's.
+setCustomExtension("luau", "lua");
+setCustomExtension("xsl", "xml");
+setCustomExtension("xslt", "xml");
+
 /** The native lexer ID for a @pierre/highlights language name, or 0 (plain text). */
 export function highlightLanguageId(language: string): number {
   return Object.hasOwn(HIGHLIGHTS_LANGUAGE_IDS, language) ? HIGHLIGHTS_LANGUAGE_IDS[language]! : 0;
 
@@ -1,9 +1,19 @@
 import { getFiletypeFromFileName } from "@pierre/diffs";
-import { expect, it } from "vite-plus/test";
+import { describe, expect, it } from "vite-plus/test";
 
 import { getSyntaxHighlighterPromise } from "./syntaxHighlighting";
 import { DIFF_THEME_NAMES } from "./diffRendering";
 
+describe("dialect file types", () => {
+  it.each([
+    ["src/init.luau", "lua"],
+    ["styles/page.xsl", "xml"],
+    ["styles/page.xslt", "xml"],
+  ])("%s highlights as %s", (path, language) => {
+    expect(getFiletypeFromFileName(path)).toBe(language);
+  });
+});
+
 it.each([
   "asm",
   "assembly",
 
@@ -1,13 +1,19 @@
 import "@t3tools/client-runtime/assembly-highlighting";
 import {
   getSharedHighlighter,
+  setCustomExtension,
   type DiffsHighlighter,
   type HighlighterTypes,
   type SupportedLanguages,
 } from "@pierre/diffs";
 
 import { DIFF_THEME_NAMES } from "./diffRendering";
 
+// Highlights has no lexer for these dialects, so they use their base language's.
+setCustomExtension("luau", "lua");
+setCustomExtension("xsl", "xml");
+setCustomExtension("xslt", "xml");
+
 /**
  * Highlights' hand-written WASM lexers tokenize large files 10-40x faster than
  * Shiki and never backtrack like Shiki's JS regex engine. Only one highlighter