diff --git a/src/textual/_animator.py b/src/textual/_animator.py
index 6ace3dcff..6fbbe890e 100644
--- a/src/textual/_animator.py
+++ b/src/textual/_animator.py
@@ -11,13 +11,9 @@ from . import _clock
 from ._callback import invoke
 from ._easing import DEFAULT_EASING, EASING
 from ._types import CallbackType
+from ._typing import Protocol, runtime_checkable
 from .timer import Timer
 
-if sys.version_info >= (3, 8):
-    from typing import Protocol, runtime_checkable
-else:  # pragma: no cover
-    from typing_extensions import Protocol, runtime_checkable
-
 if TYPE_CHECKING:
     from textual.app import App
 
diff --git a/src/textual/_compositor.py b/src/textual/_compositor.py
index e19762189..5d6c4f0eb 100644
--- a/src/textual/_compositor.py
+++ b/src/textual/_compositor.py
@@ -59,7 +59,7 @@ class MapGeometry(NamedTuple):
 
 
 # Maps a widget on to its geometry (information that describes its position in the composition)
-CompositorMap: TypeAlias = "dict[Widget, MapGeometry]"
+CompositorMap: TypeAlias = dict[Widget, MapGeometry]
 
 
 @rich.repr.auto(angular=True)
diff --git a/src/textual/_layout_resolve.py b/src/textual/_layout_resolve.py
index 156550114..a9d209034 100644
--- a/src/textual/_layout_resolve.py
+++ b/src/textual/_layout_resolve.py
@@ -1,15 +1,10 @@
 from __future__ import annotations
 
 from dataclasses import dataclass
-import sys
 from fractions import Fraction
-from typing import cast, Sequence
+from typing import Sequence, cast
 
-
-if sys.version_info >= (3, 8):
-    from typing import Protocol
-else:
-    from typing_extensions import Protocol  # pragma: no cover
+from ._typing import Protocol
 
 
 class EdgeProtocol(Protocol):
diff --git a/src/textual/_resolve.py b/src/textual/_resolve.py
index a52578704..1c5fe73df 100644
--- a/src/textual/_resolve.py
+++ b/src/textual/_resolve.py
@@ -1,10 +1,10 @@
 from __future__ import annotations
 
-import sys
 from fractions import Fraction
 from itertools import accumulate
-from typing import cast, Sequence, TYPE_CHECKING
+from typing import TYPE_CHECKING, Sequence, cast
 
+from ._typing import Literal
 from .box_model import BoxModel
 from .css.scalar import Scalar
 from .geometry import Size
@@ -13,12 +13,6 @@ if TYPE_CHECKING:
     from .widget import Widget
 
 
-if sys.version_info >= (3, 8):
-    from typing import Literal
-else:
-    from typing_extensions import Literal
-
-
 def resolve(
     dimensions: Sequence[Scalar],
     total: int,
diff --git a/src/textual/_types.py b/src/textual/_types.py
index 52b2390cf..56a3efd33 100644
--- a/src/textual/_types.py
+++ b/src/textual/_types.py
@@ -7,7 +7,6 @@ from ._typing import Protocol
 
 if TYPE_CHECKING:
     from .message import Message
-    from .strip import Strip
 
 
 class MessageTarget(Protocol):
diff --git a/src/textual/_typing.py b/src/textual/_typing.py
index 340137d3e..31e295249 100644
--- a/src/textual/_typing.py
+++ b/src/textual/_typing.py
@@ -6,8 +6,15 @@ else:  # pragma: no cover
     from typing_extensions import TypeAlias
 
 if sys.version_info >= (3, 8):
-    from typing import Final, Literal, Protocol, TypedDict
+    from typing import Final, Literal, Protocol, TypedDict, runtime_checkable
 else:
-    from typing_extensions import Final, Literal, Protocol, TypedDict
+    from typing_extensions import Final, Literal, Protocol, TypedDict, runtime_checkable
 
-__all__ = ["TypeAlias", "Final", "Literal", "Protocol", "TypedDict"]
+__all__ = [
+    "Final",
+    "Literal",
+    "Protocol",
+    "runtime_checkable",
+    "TypeAlias",
+    "TypedDict",
+]
diff --git a/src/textual/app.py b/src/textual/app.py
index 6abf74839..4ed18dc0b 100644
--- a/src/textual/app.py
+++ b/src/textual/app.py
@@ -51,6 +51,7 @@ from ._event_broker import NoHandler, extract_handler_actions
 from ._filter import LineFilter, Monochrome
 from ._path import _make_path_object_relative
 from ._typing import Final, TypeAlias
+from ._wait import wait_for_idle
 from .actions import SkipAction
 from .await_remove import AwaitRemove
 from .binding import Binding, Bindings
@@ -68,7 +69,6 @@ from .messages import CallbackType
 from .reactive import Reactive
 from .renderables.blank import Blank
 from .screen import Screen
-from ._wait import wait_for_idle
 from .widget import AwaitMount, Widget
 
 if TYPE_CHECKING:
diff --git a/src/textual/css/constants.py b/src/textual/css/constants.py
index 48aa85147..255690825 100644
--- a/src/textual/css/constants.py
+++ b/src/textual/css/constants.py
@@ -7,8 +7,8 @@ from .._typing import Final
 if typing.TYPE_CHECKING:
     from .types import EdgeType
 
-VALID_VISIBILITY: Final = {"visible", "hidden"}
-VALID_DISPLAY: Final = {"block", "none"}
+VALID_VISIBILITY: Final[set[str]] = {"visible", "hidden"}
+VALID_DISPLAY: Final[set[str]] = {"block", "none"}
 VALID_BORDER: Final[set[EdgeType]] = {
     "none",
     "hidden",
@@ -26,16 +26,23 @@ VALID_BORDER: Final[set[EdgeType]] = {
     "tall",
     "wide",
 }
-VALID_EDGE: Final = {"top", "right", "bottom", "left"}
-VALID_LAYOUT: Final = {"vertical", "horizontal", "grid"}
+VALID_EDGE: Final[set[str]] = {"top", "right", "bottom", "left"}
+VALID_LAYOUT: Final[set[str]] = {"vertical", "horizontal", "grid"}
 
-VALID_BOX_SIZING: Final = {"border-box", "content-box"}
-VALID_OVERFLOW: Final = {"scroll", "hidden", "auto"}
-VALID_ALIGN_HORIZONTAL: Final = {"left", "center", "right"}
-VALID_ALIGN_VERTICAL: Final = {"top", "middle", "bottom"}
-VALID_TEXT_ALIGN: Final = {"start", "end", "left", "right", "center", "justify"}
-VALID_SCROLLBAR_GUTTER: Final = {"auto", "stable"}
-VALID_STYLE_FLAGS: Final = {
+VALID_BOX_SIZING: Final[set[str]] = {"border-box", "content-box"}
+VALID_OVERFLOW: Final[set[str]] = {"scroll", "hidden", "auto"}
+VALID_ALIGN_HORIZONTAL: Final[set[str]] = {"left", "center", "right"}
+VALID_ALIGN_VERTICAL: Final[set[str]] = {"top", "middle", "bottom"}
+VALID_TEXT_ALIGN: Final[set[str]] = {
+    "start",
+    "end",
+    "left",
+    "right",
+    "center",
+    "justify",
+}
+VALID_SCROLLBAR_GUTTER: Final[set[str]] = {"auto", "stable"}
+VALID_STYLE_FLAGS: Final[set[str]] = {
     "b",
     "blink",
     "bold",
@@ -53,4 +60,5 @@ VALID_STYLE_FLAGS: Final = {
     "uu",
 }
 
-NULL_SPACING: Final = Spacing.all(0)
+
+NULL_SPACING: Final[Spacing] = Spacing.all(0)