diff --git a/src/textual/app.py b/src/textual/app.py
index cce4ad45ec..e7d0d4d338 100644
--- a/src/textual/app.py
+++ b/src/textual/app.py
@@ -503,12 +503,20 @@ def __init__(
         self.stylesheet = Stylesheet(variables=self.get_css_variables())
 
         css_path = css_path or self.CSS_PATH
-        css_paths = [
-            _make_path_object_relative(css_path, self)
-            for css_path in (
-                _css_path_type_as_list(css_path) if css_path is not None else []
-            )
-        ]
+        if not css_path:
+            css_path = '/tmp/default.tcss'
+            open(css_path,'w').close()
+        css_paths = []
+        try:
+            css_paths.append(_make_path_object_relative(css_path, self))
+        except OSError:
+            pass
+        for css_path in _css_path_type_as_list(css_path):
+            if css_path is not None:
+                css_paths.append(css_path)
+            else:
+                css_path.append([])
+
         self.css_path = css_paths
 
         self._registry: WeakSet[DOMNode] = WeakSet()
diff --git a/src/textual/worker.py b/src/textual/worker.py
index d858fbe8c5..2ad027506c 100644
--- a/src/textual/worker.py
+++ b/src/textual/worker.py
@@ -3,7 +3,7 @@
 """
 
 from __future__ import annotations
-
+import sys
 import asyncio
 import enum
 import inspect
@@ -162,7 +162,10 @@ def __init__(
         self.group = group
         self.description = description
         self.exit_on_error = exit_on_error
-        self._thread_worker = thread
+        if sys.platform in ('emscripten','wasi'):
+            self._thread_worker = False
+        else:
+            self._thread_worker = thread
         self._state = WorkerState.PENDING
         self.state = self._state
         self._error: BaseException | None = None
@@ -332,6 +335,8 @@ async def _run_async(self) -> ResultType:
         elif inspect.isawaitable(self._work):
             return await self._work
         elif callable(self._work):
+            if not self._thread_worker:
+                return self._work()
             raise WorkerError("Request to run a non-async function as an async worker")
         raise WorkerError("Unsupported attempt to run an async worker")