fix: make htmlFallback more permissive (#15059)

Author: sapphi-red

packages/vite/src/node/server/middlewares/htmlFallback.ts

@@ -14,11 +14,10 @@ export function htmlFallbackMiddleware(
     if (
       // Only accept GET or HEAD
       (req.method !== 'GET' && req.method !== 'HEAD') ||
-      // Ignore JSON requests
-      req.headers.accept?.includes('application/json') ||
       // Require Accept: text/html or */*
       !(
         req.headers.accept === undefined || // equivalent to `Accept: */*`
+        req.headers.accept === '' || // equivalent to `Accept: */*`
         req.headers.accept.includes('text/html') ||
         req.headers.accept.includes('*/*')
       )

playground/html/__tests__/html.spec.ts

@@ -414,3 +414,19 @@ test('html serve behavior', async () => {
   expect(bothSlashIndexHtml.status).toBe(200)
   expect(await bothSlashIndexHtml.text()).toContain('both/index.html')
 })
+
+test('html fallback works non browser accept header', async () => {
+  expect((await fetch(viteTestUrl, { headers: { Accept: '' } })).status).toBe(
+    200,
+  )
+  // defaults to "Accept: */*"
+  expect((await fetch(viteTestUrl)).status).toBe(200)
+  // wait-on uses axios and axios sends this accept header
+  expect(
+    (
+      await fetch(viteTestUrl, {
+        headers: { Accept: 'application/json, text/plain, */*' },
+      })
+    ).status,
+  ).toBe(200)
+})