diff --git a/docs/examples/introduction/stopwatch.css b/docs/examples/introduction/stopwatch.css
index 472dccc89..93678369c 100644
--- a/docs/examples/introduction/stopwatch.css
+++ b/docs/examples/introduction/stopwatch.css
@@ -51,4 +51,3 @@ Button {
 .started #reset {
     visibility: hidden
 }
-
diff --git a/src/textual/_compositor.py b/src/textual/_compositor.py
index 3fab34aeb..8d5c299b7 100644
--- a/src/textual/_compositor.py
+++ b/src/textual/_compositor.py
@@ -183,6 +183,9 @@ class Compositor:
         # Note this may be a superset of self.map.keys() as some widgets may be invisible for various reasons
         self.widgets: set[Widget] = set()
 
+        # A lazy cache of visible (on screen) widgets
+        self._visible_widgets: set[Widget] | None = set()
+
         # The top level widget
         self.root: Widget | None = None
 
@@ -269,6 +272,7 @@ class Compositor:
         # Replace map and widgets
         self.map = map
         self.widgets = widgets
+        self._visible_widgets = None
 
         # Get a map of regions
         self.regions = {
@@ -305,6 +309,22 @@ class Compositor:
             resized=resized_widgets,
         )
 
+    @property
+    def visible_widgets(self) -> set[Widget]:
+        """Get a set of visible widgets.
+
+        Returns:
+            set[Widget]: Widgets in the screen.
+        """
+        if self._visible_widgets is None:
+            in_screen = self.size.region.__contains__
+            self._visible_widgets = {
+                widget
+                for widget, (region, clip) in self.regions.items()
+                if in_screen(region)
+            }
+        return self._visible_widgets
+
     def _arrange_root(
         self, root: Widget, size: Size
     ) -> tuple[CompositorMap, set[Widget]]:
diff --git a/src/textual/app.py b/src/textual/app.py
index 38afb8677..f56f39052 100644
--- a/src/textual/app.py
+++ b/src/textual/app.py
@@ -224,7 +224,7 @@ class App(Generic[ReturnType], DOMNode):
         self.design = DEFAULT_COLORS
 
         self.stylesheet = Stylesheet(variables=self.get_css_variables())
-        self._require_stylesheet_update = False
+        self._require_stylesheet_update: set[DOMNode] = set()
         self.css_path = css_path or self.CSS_PATH
 
         self._registry: WeakSet[DOMNode] = WeakSet()
@@ -714,13 +714,18 @@ class App(Generic[ReturnType], DOMNode):
         """
         return self.screen.get_child(id)
 
-    def update_styles(self) -> None:
+    def update_styles(self, node: DOMNode | None = None) -> None:
         """Request update of styles.
 
         Should be called whenever CSS classes / pseudo classes change.
 
         """
-        self._require_stylesheet_update = True
+        self._require_stylesheet_update.add(self.screen if node is None else node)
+        self.check_idle()
+
+    def update_visible_styles(self) -> None:
+        """Update visible styles only."""
+        self._require_stylesheet_update.update(self.screen.visible_widgets)
         self.check_idle()
 
     def mount(self, *anon_widgets: Widget, **widgets: Widget) -> None:
@@ -1137,16 +1142,21 @@ class App(Generic[ReturnType], DOMNode):
 
         self.set_timer(screenshot_timer, on_screenshot, name="screenshot timer")
 
-    def on_mount(self) -> None:
+    def _on_mount(self) -> None:
         widgets = self.compose()
         if widgets:
             self.mount_all(widgets)
 
-    async def on_idle(self) -> None:
+    def _on_idle(self) -> None:
         """Perform actions when there are no messages in the queue."""
         if self._require_stylesheet_update:
-            self._require_stylesheet_update = False
-            self.stylesheet.update(self, animate=True)
+            nodes: set[DOMNode] = {
+                child
+                for node in self._require_stylesheet_update
+                for child in node.walk_children()
+            }
+            self._require_stylesheet_update.clear()
+            self.stylesheet.update_nodes(nodes, animate=True)
 
     def _register_child(self, parent: DOMNode, child: Widget) -> bool:
         if child not in self._registry:
diff --git a/src/textual/css/model.py b/src/textual/css/model.py
index cc960eedb..c5efebc15 100644
--- a/src/textual/css/model.py
+++ b/src/textual/css/model.py
@@ -159,9 +159,13 @@ class RuleSet:
     selector_set: list[SelectorSet] = field(default_factory=list)
     styles: Styles = field(default_factory=Styles)
     errors: list[tuple[Token, str]] = field(default_factory=list)
-    classes: set[str] = field(default_factory=set)
+
     is_default_rules: bool = False
     tie_breaker: int = 0
+    selector_names: set[str] = field(default_factory=set)
+
+    def __hash__(self):
+        return id(self)
 
     @classmethod
     def _selector_to_css(cls, selectors: list[Selector]) -> str:
@@ -195,11 +199,37 @@ class RuleSet:
     def _post_parse(self) -> None:
         """Called after the RuleSet is parsed."""
         # Build a set of the class names that have been updated
-        update = self.classes.update
+
         class_type = SelectorType.CLASS
+        id_type = SelectorType.ID
+        type_type = SelectorType.TYPE
+        universal_type = SelectorType.UNIVERSAL
+
+        update_selectors = self.selector_names.update
+
         for selector_set in self.selector_set:
-            update(
+            update_selectors(
+                "*"
+                for selector in selector_set.selectors
+                if selector.type == universal_type
+            )
+            update_selectors(
                 selector.name
                 for selector in selector_set.selectors
+                if selector.type == type_type
+            )
+            update_selectors(
+                f".{selector.name}"
+                for selector in selector_set.selectors
                 if selector.type == class_type
             )
+            update_selectors(
+                f"#{selector.name}"
+                for selector in selector_set.selectors
+                if selector.type == id_type
+            )
+            update_selectors(
+                f":{pseudo_class}"
+                for selector in selector_set.selectors
+                for pseudo_class in selector.pseudo_classes
+            )
diff --git a/src/textual/css/stylesheet.py b/src/textual/css/stylesheet.py
index 0ea08020c..849688c42 100644
--- a/src/textual/css/stylesheet.py
+++ b/src/textual/css/stylesheet.py
@@ -4,10 +4,10 @@ import os
 from collections import defaultdict
 from operator import itemgetter
 from pathlib import Path, PurePath
-from typing import cast, Iterable, NamedTuple
+from typing import Iterable, NamedTuple, cast
 
 import rich.repr
-from rich.console import RenderableType, RenderResult, Console, ConsoleOptions
+from rich.console import Console, ConsoleOptions, RenderableType, RenderResult
 from rich.markup import render
 from rich.padding import Padding
 from rich.panel import Panel
@@ -15,17 +15,18 @@ from rich.style import Style
 from rich.syntax import Syntax
 from rich.text import Text
 
-from textual.widget import Widget
+from .. import messages
+from .._profile import timer
+from ..dom import DOMNode
+from ..widget import Widget
 from .errors import StylesheetError
 from .match import _check_selectors
 from .model import RuleSet
 from .parse import parse
 from .styles import RulesMap, Styles
-from .tokenize import tokenize_values, Token
+from .tokenize import Token, tokenize_values
 from .tokenizer import TokenError
 from .types import Specificity3, Specificity6
-from ..dom import DOMNode
-from .. import messages
 
 
 class StylesheetParseError(StylesheetError):
@@ -135,6 +136,7 @@ class CssSource(NamedTuple):
 class Stylesheet:
     def __init__(self, *, variables: dict[str, str] | None = None) -> None:
         self._rules: list[RuleSet] = []
+        self._rules_map: dict[str, list[RuleSet]] | None = None
         self.variables = variables or {}
         self.source: dict[str, CssSource] = {}
         self._require_parse = False
@@ -144,12 +146,32 @@ class Stylesheet:
 
     @property
     def rules(self) -> list[RuleSet]:
+        """List of rule sets.
+
+        Returns:
+            list[RuleSet]: List of rules sets for this stylesheet.
+        """
         if self._require_parse:
             self.parse()
             self._require_parse = False
         assert self._rules is not None
         return self._rules
 
+    @property
+    def rules_map(self) -> dict[str, list[RuleSet]]:
+        """Structure that maps a selector on to a list of rules.
+
+        Returns:
+            dict[str, list[RuleSet]]: Mapping of selector to rule sets.
+        """
+        if self._rules_map is None:
+            rules_map: dict[str, list[RuleSet]] = defaultdict(list)
+            for rule in self.rules:
+                for name in rule.selector_names:
+                    rules_map[name].append(rule)
+            self._rules_map = dict(rules_map)
+        return self._rules_map
+
     @property
     def css(self) -> str:
         return "\n\n".join(rule_set.css for rule_set in self.rules)
@@ -283,6 +305,7 @@ class Stylesheet:
             add_rules(css_rules)
         self._rules = rules
         self._require_parse = False
+        self._rules_map = None
 
     def reparse(self) -> None:
         """Re-parse source, applying new variables.
@@ -300,15 +323,24 @@ class Stylesheet:
             )
         stylesheet.parse()
         self._rules = stylesheet.rules
+        self._rules_map = None
         self.source = stylesheet.source
 
     @classmethod
-    def _check_rule(cls, rule: RuleSet, node: DOMNode) -> Iterable[Specificity3]:
+    def _check_rule(
+        cls, rule: RuleSet, css_path_nodes: list[DOMNode]
+    ) -> Iterable[Specificity3]:
         for selector_set in rule.selector_set:
-            if _check_selectors(selector_set.selectors, node.css_path_nodes):
+            if _check_selectors(selector_set.selectors, css_path_nodes):
                 yield selector_set.specificity
 
-    def apply(self, node: DOMNode, animate: bool = False) -> None:
+    def apply(
+        self,
+        node: DOMNode,
+        *,
+        limit_rules: set[RuleSet] | None = None,
+        animate: bool = False,
+    ) -> None:
         """Apply the stylesheet to a DOM node.
 
         Args:
@@ -319,32 +351,34 @@ class Stylesheet:
                 rule will be applied.
             animate (bool, optional): Animate changed rules. Defaults to ``False``.
         """
-
-        # TODO: Need to optimize to make applying stylesheet more efficient
-        # I think we can pre-calculate which rules may be applicable to a given node
-
         # Dictionary of rule attribute names e.g. "text_background" to list of tuples.
         # The tuples contain the rule specificity, and the value for that rule.
         # We can use this to determine, for a given rule, whether we should apply it
         # or not by examining the specificity. If we have two rules for the
         # same attribute, then we can choose the most specific rule and use that.
-        rule_attributes: dict[str, list[tuple[Specificity6, object]]]
-        rule_attributes = {}
+        rule_attributes: defaultdict[str, list[tuple[Specificity6, object]]]
+        rule_attributes = defaultdict(list)
 
         _check_rule = self._check_rule
+        css_path_nodes = node.css_path_nodes
 
+        rules: Iterable[RuleSet]
+        if limit_rules:
+            rules = [rule for rule in reversed(self.rules) if rule in limit_rules]
+        else:
+            rules = reversed(self.rules)
         # Collect the rules defined in the stylesheet
-        for rule in reversed(self.rules):
+        for rule in rules:
             is_default_rules = rule.is_default_rules
             tie_breaker = rule.tie_breaker
-            for base_specificity in _check_rule(rule, node):
+            for base_specificity in _check_rule(rule, css_path_nodes):
                 for key, rule_specificity, value in rule.styles.extract_rules(
                     base_specificity, is_default_rules, tie_breaker
                 ):
-                    rule_attributes.setdefault(key, []).append(
-                        (rule_specificity, value)
-                    )
+                    rule_attributes[key].append((rule_specificity, value))
 
+        if not rule_attributes:
+            return
         # For each rule declared for this node, keep only the most specific one
         get_first_item = itemgetter(0)
         node_rules: RulesMap = cast(
@@ -354,7 +388,6 @@ class Stylesheet:
                 for name, specificity_rules in rule_attributes.items()
             },
         )
-
         self.replace_rules(node, node_rules, animate=animate)
 
         node._component_styles.clear()
@@ -381,7 +414,7 @@ class Stylesheet:
         base_styles = styles.base
 
         # Styles currently used on new rules
-        modified_rule_keys = {*base_styles.get_rules().keys(), *rules.keys()}
+        modified_rule_keys = base_styles.get_rules().keys() | rules.keys()
         # Current render rules (missing rules are filled with default)
         current_render_rules = styles.get_render_rules()
 
@@ -434,10 +467,34 @@ class Stylesheet:
         node.post_message_no_wait(messages.StylesUpdated(sender=node))
 
     def update(self, root: DOMNode, animate: bool = False) -> None:
-        """Update a node and its children."""
+        """Update styles on node and its children.
+
+        Args:
+            root (DOMNode): Root note to update.
+            animate (bool, optional): Enable CSS animation. Defaults to False.
+        """
+
+        self.update_nodes(root.walk_children(), animate=animate)
+
+    def update_nodes(self, nodes: Iterable[DOMNode], animate: bool = False) -> None:
+        """Update styles for nodes.
+
+        Args:
+            nodes (DOMNode): Nodes to update.
+            animate (bool, optional): Enable CSS animation. Defaults to False.
+        """
+
+        rules_map = self.rules_map
         apply = self.apply
-        for node in root.walk_children():
-            apply(node, animate=animate)
+
+        for node in nodes:
+            rules = {
+                rule
+                for name in node._selector_names
+                if name in rules_map
+                for rule in rules_map[name]
+            }
+            apply(node, limit_rules=rules, animate=animate)
             if isinstance(node, Widget) and node.is_scrollable:
                 if node.show_vertical_scrollbar:
                     apply(node.vertical_scrollbar)
diff --git a/src/textual/devtools/client.py b/src/textual/devtools/client.py
index 63c59f3d8..802bd686a 100644
--- a/src/textual/devtools/client.py
+++ b/src/textual/devtools/client.py
@@ -163,6 +163,7 @@ class DevtoolsClient:
                 if isinstance(log, str):
                     await websocket.send_str(log)
                 else:
+                    assert isinstance(log, bytes)
                     await websocket.send_bytes(log)
                 log_queue.task_done()
 
diff --git a/src/textual/dom.py b/src/textual/dom.py
index f2c3ca31e..984ac8f16 100644
--- a/src/textual/dom.py
+++ b/src/textual/dom.py
@@ -87,6 +87,7 @@ class DOMNode(MessagePump):
 
         self._auto_refresh: float | None = None
         self._auto_refresh_timer: Timer | None = None
+        self._css_types = {cls.__name__ for cls in self._css_bases(self.__class__)}
 
         super().__init__()
 
@@ -311,6 +312,23 @@ class DOMNode(MessagePump):
             append(node)
         return result[::-1]
 
+    @property
+    def _selector_names(self) -> list[str]:
+        """Get a set of selectors applicable to this widget.
+
+        Returns:
+            set[str]: Set of selector names.
+        """
+        selectors: list[str] = [
+            "*",
+            *(f".{class_name}" for class_name in self._classes),
+            *(f":{class_name}" for class_name in self.get_pseudo_classes()),
+            *self._css_types,
+        ]
+        if self._id is not None:
+            selectors.append(f"#{self._id}")
+        return selectors
+
     @property
     def display(self) -> bool:
         """
@@ -699,7 +717,7 @@ class DOMNode(MessagePump):
         if old_classes == self._classes:
             return
         try:
-            self.app.stylesheet.update(self.app, animate=True)
+            self.app.update_styles(self)
         except NoActiveAppError:
             pass
 
@@ -715,7 +733,7 @@ class DOMNode(MessagePump):
         if old_classes == self._classes:
             return
         try:
-            self.app.stylesheet.update(self.app, animate=True)
+            self.app.update_styles(self)
         except NoActiveAppError:
             pass
 
@@ -731,7 +749,7 @@ class DOMNode(MessagePump):
         if old_classes == self._classes:
             return
         try:
-            self.app.stylesheet.update(self.app, animate=True)
+            self.app.update_styles(self)
         except NoActiveAppError:
             pass
 
diff --git a/src/textual/screen.py b/src/textual/screen.py
index 2cbd12440..303c76ea0 100644
--- a/src/textual/screen.py
+++ b/src/textual/screen.py
@@ -63,6 +63,16 @@ class Screen(Widget):
             )
         return self._update_timer
 
+    @property
+    def widgets(self) -> list[Widget]:
+        """Get all widgets."""
+        return list(self._compositor.map.keys())
+
+    @property
+    def visible_widgets(self) -> list[Widget]:
+        """Get a list of visible widgets."""
+        return list(self._compositor.visible_widgets)
+
     def watch_dark(self, dark: bool) -> None:
         pass
 
diff --git a/src/textual/widget.py b/src/textual/widget.py
index 32cda2fb2..86a46dc8e 100644
--- a/src/textual/widget.py
+++ b/src/textual/widget.py
@@ -1227,11 +1227,11 @@ class Widget(DOMNode):
 
     def watch_mouse_over(self, value: bool) -> None:
         """Update from CSS if mouse over state changes."""
-        self.app.update_styles()
+        self.app.update_styles(self)
 
     def watch_has_focus(self, value: bool) -> None:
         """Update from CSS if has focus state changes."""
-        self.app.update_styles()
+        self.app.update_styles(self)
 
     def size_updated(
         self, size: Size, virtual_size: Size, container_size: Size