Add clarifications.

tests/snapshot_tests/snapshot_apps/hot_reloading_app.css

@@ -0,0 +1 @@
+/* This file is purposefully empty. */

tests/snapshot_tests/snapshot_apps/hot_reloading_app.py

@@ -7,6 +7,9 @@ from textual.widgets import Label
 
 CSS_PATH = (Path(__file__) / "../hot_reloading_app.css").resolve()
 
+# Write some CSS to the file before the app loads.
+# Then, the test will clear all the CSS to see if the
+# hot reloading applies the changes correctly.
 CSS_PATH.write_text(
     """
 Container {

tests/snapshot_tests/test_snapshots.py

@@ -343,10 +343,12 @@ def test_scrollbar_thumb_height(snap_compare):
 
 
 def test_css_hot_reloading(snap_compare):
+    """Regression test for https://github.com/Textualize/textual/issues/2063."""
+
     async def run_before(pilot):
         css_file = pilot.app.CSS_PATH
         with open(css_file, "w") as f:
-            f.write("")  # Clear all the CSS.
+            f.write("/* This file is purposefully empty. */\n")  # Clear all the CSS.
         await pilot.app._on_css_change()
 
     assert snap_compare(