diff --git a/changes/2050.feature.rst b/changes/2050.feature.rst
new file mode 100644
index 0000000000..e1632062c5
--- /dev/null
+++ b/changes/2050.feature.rst
@@ -0,0 +1 @@
+An implementation of ActivityIndicator was added to the Web backend.
diff --git a/core/src/toga/style/pack.py b/core/src/toga/style/pack.py
index 6208d1f604..0fd90facb2 100644
--- a/core/src/toga/style/pack.py
+++ b/core/src/toga/style/pack.py
@@ -767,7 +767,7 @@ def __css__(self):
         if (self.width == NONE and self.direction == ROW) or (
             self.height == NONE and self.direction == COLUMN
         ):
-            css.append(f"flex: {self.flex} 0 0;")
+            css.append(f"flex: {self.flex} 0 auto;")
 
         # width/flex
         if self.width != NONE:
diff --git a/core/tests/style/pack/test_css.py b/core/tests/style/pack/test_css.py
index 6562754230..f204c79d1e 100644
--- a/core/tests/style/pack/test_css.py
+++ b/core/tests/style/pack/test_css.py
@@ -32,40 +32,40 @@
         # Empty definition
         pytest.param(
             Pack(),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="empty",
         ),
         # Display
         pytest.param(
             Pack(display=PACK),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="display-pack",
         ),
         pytest.param(
             Pack(display=NONE),
-            "display: none; flex-direction: row; flex: 0.0 0 0;",
+            "display: none; flex-direction: row; flex: 0.0 0 auto;",
             id="display-none",
         ),
         # Visibility
         pytest.param(
             Pack(visibility=VISIBLE),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="visibility-visible",
         ),
         pytest.param(
             Pack(visibility=HIDDEN),
-            "visibility: hidden; flex-direction: row; flex: 0.0 0 0;",
+            "visibility: hidden; flex-direction: row; flex: 0.0 0 auto;",
             id="visibility-hidden",
         ),
         # Direction
         pytest.param(
             Pack(direction=ROW),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="direction-row",
         ),
         pytest.param(
             Pack(direction=COLUMN),
-            "flex-direction: column; flex: 0.0 0 0;",
+            "flex-direction: column; flex: 0.0 0 auto;",
             id="direction-column",
         ),
         # Width
@@ -86,12 +86,12 @@
         ),
         pytest.param(
             Pack(width=NONE),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="width-none",
         ),
         pytest.param(
             Pack(width=NONE, flex=5),
-            "flex-direction: row; flex: 5.0 0 0;",
+            "flex-direction: row; flex: 5.0 0 auto;",
             id="width-none-flex",
         ),
         pytest.param(
@@ -111,88 +111,88 @@
         ),
         pytest.param(
             Pack(direction=ROW, width=NONE),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="width-row-none",
         ),
         pytest.param(
             Pack(direction=ROW, width=NONE, flex=5),
-            "flex-direction: row; flex: 5.0 0 0;",
+            "flex-direction: row; flex: 5.0 0 auto;",
             id="width-row-none-flex",
         ),
         pytest.param(
             Pack(direction=COLUMN, width=42),
-            "flex-direction: column; flex: 0.0 0 0; width: 42px;",
+            "flex-direction: column; flex: 0.0 0 auto; width: 42px;",
             id="width-column-explicit",
         ),
         pytest.param(
             Pack(direction=COLUMN, width=42, flex=5),
-            "flex-direction: column; flex: 5.0 0 0; width: 42px;",
+            "flex-direction: column; flex: 5.0 0 auto; width: 42px;",
             id="width-column-explicit-flex",
         ),
         pytest.param(
             Pack(direction=COLUMN, width=0),
-            "flex-direction: column; flex: 0.0 0 0; width: 0px;",
+            "flex-direction: column; flex: 0.0 0 auto; width: 0px;",
             id="width-column-0",
         ),
         pytest.param(
             Pack(direction=COLUMN, width=NONE),
-            "flex-direction: column; flex: 0.0 0 0;",
+            "flex-direction: column; flex: 0.0 0 auto;",
             id="width-column-none",
         ),
         pytest.param(
             Pack(direction=COLUMN, width=NONE, flex=5),
-            "flex-direction: column; flex: 5.0 0 0;",
+            "flex-direction: column; flex: 5.0 0 auto;",
             id="width-column-none-flex",
         ),
         # Height
         pytest.param(
             Pack(height=42),
-            "flex-direction: row; flex: 0.0 0 0; height: 42px;",
+            "flex-direction: row; flex: 0.0 0 auto; height: 42px;",
             id="height-explicit",
         ),
         pytest.param(
             Pack(height=42, flex=5),
-            "flex-direction: row; flex: 5.0 0 0; height: 42px;",
+            "flex-direction: row; flex: 5.0 0 auto; height: 42px;",
             id="height-explicit-flex",
         ),
         pytest.param(
             Pack(height=0),
-            "flex-direction: row; flex: 0.0 0 0; height: 0px;",
+            "flex-direction: row; flex: 0.0 0 auto; height: 0px;",
             id="height-0",
         ),
         pytest.param(
             Pack(height=NONE),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="height-none",
         ),
         pytest.param(
             Pack(height=NONE, flex=5),
-            "flex-direction: row; flex: 5.0 0 0;",
+            "flex-direction: row; flex: 5.0 0 auto;",
             id="height-none-flex",
         ),
         pytest.param(
             Pack(direction=ROW, height=42),
-            "flex-direction: row; flex: 0.0 0 0; height: 42px;",
+            "flex-direction: row; flex: 0.0 0 auto; height: 42px;",
             id="height-row-explicit",
         ),
         pytest.param(
             Pack(direction=ROW, height=42, flex=5),
-            "flex-direction: row; flex: 5.0 0 0; height: 42px;",
+            "flex-direction: row; flex: 5.0 0 auto; height: 42px;",
             id="height-row-explicit",
         ),
         pytest.param(
             Pack(direction=ROW, height=0),
-            "flex-direction: row; flex: 0.0 0 0; height: 0px;",
+            "flex-direction: row; flex: 0.0 0 auto; height: 0px;",
             id="height-row-0",
         ),
         pytest.param(
             Pack(direction=ROW, height=NONE),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="height-row-none",
         ),
         pytest.param(
             Pack(direction=ROW, height=NONE, flex=5),
-            "flex-direction: row; flex: 5.0 0 0;",
+            "flex-direction: row; flex: 5.0 0 auto;",
             id="height-row-none",
         ),
         pytest.param(
@@ -212,117 +212,117 @@
         ),
         pytest.param(
             Pack(direction=COLUMN, height=NONE),
-            "flex-direction: column; flex: 0.0 0 0;",
+            "flex-direction: column; flex: 0.0 0 auto;",
             id="height-column-none",
         ),
         pytest.param(
             Pack(direction=COLUMN, height=NONE, flex=5),
-            "flex-direction: column; flex: 5.0 0 0;",
+            "flex-direction: column; flex: 5.0 0 auto;",
             id="height-column-none",
         ),
         # Alignment - default layout
         pytest.param(
             Pack(alignment=LEFT),
-            "flex-direction: row; flex: 0.0 0 0; align-items: start;",
+            "flex-direction: row; flex: 0.0 0 auto; align-items: start;",
             id="alignment-left",
         ),
         pytest.param(
             Pack(alignment=RIGHT),
-            "flex-direction: row; flex: 0.0 0 0; align-items: end;",
+            "flex-direction: row; flex: 0.0 0 auto; align-items: end;",
             id="alignment-right",
         ),
         pytest.param(  # alignment is ignored
             Pack(alignment=TOP),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="alignment-top",
         ),
         pytest.param(  # alignment is ignored
             Pack(alignment=BOTTOM),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="alignment-bottom",
         ),
         pytest.param(
             Pack(alignment=CENTER),
-            "flex-direction: row; flex: 0.0 0 0; align-items: center;",
+            "flex-direction: row; flex: 0.0 0 auto; align-items: center;",
             id="alignment-center",
         ),
         # Alignment - row layout
         pytest.param(
             Pack(direction=ROW, alignment=LEFT),
-            "flex-direction: row; flex: 0.0 0 0; align-items: start;",
+            "flex-direction: row; flex: 0.0 0 auto; align-items: start;",
             id="row-alignment-left",
         ),
         pytest.param(
             Pack(direction=ROW, alignment=RIGHT),
-            "flex-direction: row; flex: 0.0 0 0; align-items: end;",
+            "flex-direction: row; flex: 0.0 0 auto; align-items: end;",
             id="row-alignment-right",
         ),
         pytest.param(  # alignment is ignored
             Pack(direction=ROW, alignment=TOP),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="row-alignment-top",
         ),
         pytest.param(  # alignment is ignored
             Pack(direction=ROW, alignment=BOTTOM),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="row-alignment-bottom",
         ),
         pytest.param(
             Pack(direction=ROW, alignment=CENTER),
-            "flex-direction: row; flex: 0.0 0 0; align-items: center;",
+            "flex-direction: row; flex: 0.0 0 auto; align-items: center;",
             id="row-alignment-center",
         ),
         # Alignment - column layout
         pytest.param(  # alignment is ignored
             Pack(direction=COLUMN, alignment=LEFT),
-            "flex-direction: column; flex: 0.0 0 0;",
+            "flex-direction: column; flex: 0.0 0 auto;",
             id="column-alignment-left",
         ),
         pytest.param(  # alignment is ignored
             Pack(direction=COLUMN, alignment=RIGHT),
-            "flex-direction: column; flex: 0.0 0 0;",
+            "flex-direction: column; flex: 0.0 0 auto;",
             id="column-alignment-right",
         ),
         pytest.param(
             Pack(direction=COLUMN, alignment=TOP),
-            "flex-direction: column; flex: 0.0 0 0; align-items: start;",
+            "flex-direction: column; flex: 0.0 0 auto; align-items: start;",
             id="column-alignment-top",
         ),
         pytest.param(
             Pack(direction=COLUMN, alignment=BOTTOM),
-            "flex-direction: column; flex: 0.0 0 0; align-items: end;",
+            "flex-direction: column; flex: 0.0 0 auto; align-items: end;",
             id="column-alignment-bottom",
         ),
         pytest.param(
             Pack(direction=COLUMN, alignment=CENTER),
-            "flex-direction: column; flex: 0.0 0 0; align-items: center;",
+            "flex-direction: column; flex: 0.0 0 auto; align-items: center;",
             id="column-alignment-center",
         ),
         # Padding
         pytest.param(
             Pack(padding_top=42),
-            "flex-direction: row; flex: 0.0 0 0; margin-top: 42px;",
+            "flex-direction: row; flex: 0.0 0 auto; margin-top: 42px;",
             id="padding-top",
         ),
         pytest.param(
             Pack(padding_bottom=42),
-            "flex-direction: row; flex: 0.0 0 0; margin-bottom: 42px;",
+            "flex-direction: row; flex: 0.0 0 auto; margin-bottom: 42px;",
             id="padding-bottom",
         ),
         pytest.param(
             Pack(padding_left=42),
-            "flex-direction: row; flex: 0.0 0 0; margin-left: 42px;",
+            "flex-direction: row; flex: 0.0 0 auto; margin-left: 42px;",
             id="padding-left",
         ),
         pytest.param(
             Pack(padding_right=42),
-            "flex-direction: row; flex: 0.0 0 0; margin-right: 42px;",
+            "flex-direction: row; flex: 0.0 0 auto; margin-right: 42px;",
             id="padding-right",
         ),
         pytest.param(
             Pack(padding=42),
             (
-                "flex-direction: row; flex: 0.0 0 0; "
+                "flex-direction: row; flex: 0.0 0 auto; "
                 "margin-top: 42px; margin-bottom: 42px; "
                 "margin-left: 42px; margin-right: 42px;"
             ),
@@ -331,127 +331,127 @@
         # Explicitly 0 padding
         pytest.param(
             Pack(padding_top=0),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="padding-top-0",
         ),
         pytest.param(
             Pack(padding_bottom=0),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="padding-bottom-0",
         ),
         pytest.param(
             Pack(padding_left=0),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="padding-left-0",
         ),
         pytest.param(
             Pack(padding_right=0),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="padding-right-0",
         ),
         pytest.param(
             Pack(padding=0),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="padding-0",
         ),
         # Color
         pytest.param(
             Pack(color=REBECCAPURPLE),
-            "flex-direction: row; flex: 0.0 0 0; color: rgb(102, 51, 153);",
+            "flex-direction: row; flex: 0.0 0 auto; color: rgb(102, 51, 153);",
             id="color",
         ),
         # Background Color
         pytest.param(
             Pack(background_color=REBECCAPURPLE),
-            "flex-direction: row; flex: 0.0 0 0; background-color: rgb(102, 51, 153);",
+            "flex-direction: row; flex: 0.0 0 auto; background-color: rgb(102, 51, 153);",
             id="background-color",
         ),
         # Text Alignment
         pytest.param(
             Pack(text_align=LEFT),
-            "flex-direction: row; flex: 0.0 0 0; text-align: left;",
+            "flex-direction: row; flex: 0.0 0 auto; text-align: left;",
             id="text-align-left",
         ),
         pytest.param(
             Pack(text_align=RIGHT),
-            "flex-direction: row; flex: 0.0 0 0; text-align: right;",
+            "flex-direction: row; flex: 0.0 0 auto; text-align: right;",
             id="text-align-right",
         ),
         pytest.param(
             Pack(text_align=CENTER),
-            "flex-direction: row; flex: 0.0 0 0; text-align: center;",
+            "flex-direction: row; flex: 0.0 0 auto; text-align: center;",
             id="text-align-center",
         ),
         pytest.param(
             Pack(text_align=JUSTIFY),
-            "flex-direction: row; flex: 0.0 0 0; text-align: justify;",
+            "flex-direction: row; flex: 0.0 0 auto; text-align: justify;",
             id="text-align-justify",
         ),
         # Text Direction
         pytest.param(
             Pack(text_direction=RTL),
-            "flex-direction: row; flex: 0.0 0 0; text-direction: rtl;",
+            "flex-direction: row; flex: 0.0 0 auto; text-direction: rtl;",
             id="text-align-rtl",
         ),
         pytest.param(
             Pack(text_direction=LTR),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="text-align-ltr",
         ),
         # Font
         pytest.param(
             Pack(font_family="Helvetica"),
-            "flex-direction: row; flex: 0.0 0 0; font-family: Helvetica;",
+            "flex-direction: row; flex: 0.0 0 auto; font-family: Helvetica;",
             id="font-family",
         ),
         pytest.param(
             Pack(font_family="Times New Roman"),
-            'flex-direction: row; flex: 0.0 0 0; font-family: "Times New Roman";',
+            'flex-direction: row; flex: 0.0 0 auto; font-family: "Times New Roman";',
             id="font-family",
         ),
         pytest.param(
             Pack(font_family=SYSTEM),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="font-family-explicit-default",
         ),
         pytest.param(
             Pack(font_style=ITALIC),
-            "flex-direction: row; flex: 0.0 0 0; font-style: italic;",
+            "flex-direction: row; flex: 0.0 0 auto; font-style: italic;",
             id="font-style",
         ),
         pytest.param(
             Pack(font_style=NORMAL),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="font-style-explicit-default",
         ),
         pytest.param(
             Pack(font_weight=BOLD),
-            "flex-direction: row; flex: 0.0 0 0; font-weight: bold;",
+            "flex-direction: row; flex: 0.0 0 auto; font-weight: bold;",
             id="font-weight",
         ),
         pytest.param(
             Pack(font_weight=NORMAL),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="font-weight-explicit-default",
         ),
         pytest.param(
             Pack(font_variant=SMALL_CAPS),
-            "flex-direction: row; flex: 0.0 0 0; font-variant: small-caps;",
+            "flex-direction: row; flex: 0.0 0 auto; font-variant: small-caps;",
             id="font-variant",
         ),
         pytest.param(
             Pack(font_variant=NORMAL),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="font-variant-explicit-default",
         ),
         pytest.param(
             Pack(font_size=42),
-            "flex-direction: row; flex: 0.0 0 0; font-size: 42pt;",
+            "flex-direction: row; flex: 0.0 0 auto; font-size: 42pt;",
             id="font-size",
         ),
         pytest.param(
             Pack(font_size=SYSTEM_DEFAULT_FONT_SIZE),
-            "flex-direction: row; flex: 0.0 0 0;",
+            "flex-direction: row; flex: 0.0 0 auto;",
             id="font-size-explicit-default",
         ),
         pytest.param(
@@ -463,7 +463,7 @@
                 font_size=42,
             ),
             (
-                "flex-direction: row; flex: 0.0 0 0; "
+                "flex-direction: row; flex: 0.0 0 auto; "
                 "font-family: Helvetica; "
                 "font-size: 42pt; "
                 "font-weight: bold; "
diff --git a/docs/reference/data/widgets_by_platform.csv b/docs/reference/data/widgets_by_platform.csv
index c0e8abb91e..933348c101 100644
--- a/docs/reference/data/widgets_by_platform.csv
+++ b/docs/reference/data/widgets_by_platform.csv
@@ -2,7 +2,7 @@ Component,Type,Component,Description,macOS,GTK,Windows,iOS,Android,Web
 Application,Core Component,:class:`~toga.App`,The application itself,|b|,|b|,|b|,|b|,|b|,|b|
 Window,Core Component,:class:`~toga.Window`,Window object,|b|,|b|,|b|,|b|,|b|,|b|
 MainWindow,Core Component,:class:`~toga.MainWindow`,Main window of the application,|b|,|b|,|b|,|b|,|b|,|b|
-ActivityIndicator,General Widget,:class:`~toga.ActivityIndicator`,A spinning activity animation,|y|,|y|,,,,
+ActivityIndicator,General Widget,:class:`~toga.ActivityIndicator`,A spinning activity animation,|y|,|y|,,,,|b|
 Button,General Widget,:class:`~toga.Button`,Basic clickable Button,|y|,|y|,|y|,|y|,|y|,|b|
 Canvas,General Widget,:class:`~toga.Canvas`,Area you can draw on,|b|,|b|,|b|,|b|,,
 DateInput,General Widget,:class:`~toga.DateInput`,A widget to select a calendar date,,,|y|,,|y|,
diff --git a/docs/reference/style/pack.rst b/docs/reference/style/pack.rst
index 57209ee5d3..3dd07c0c56 100644
--- a/docs/reference/style/pack.rst
+++ b/docs/reference/style/pack.rst
@@ -314,7 +314,7 @@ The mapping that can be used to establish the reference implementation is:
    ``display: pack``             ``display: flex``
    ``flex: <int>``               If ``direction = row`` and ``width`` is set,
                                  or ``direction = column`` and ``height`` is set,
-                                 ignore. Otherwise, ``flex: <int> 0 0``.
+                                 ignore. Otherwise, ``flex: <int> 0 auto``.
    ``font_size: <int>``          ``font-size: <int>pt``
    ``height: <value>``           ``height: <value>px`` if value is an integer;
                                  ``height: auto`` if value is ``none``.
diff --git a/web/src/toga_web/factory.py b/web/src/toga_web/factory.py
index be253f674f..8ca7af7735 100644
--- a/web/src/toga_web/factory.py
+++ b/web/src/toga_web/factory.py
@@ -8,6 +8,7 @@
 
 # from .images import Image
 from .paths import Paths
+from .widgets.activityindicator import ActivityIndicator
 from .widgets.box import Box
 from .widgets.button import Button
 
@@ -64,6 +65,7 @@ def not_implemented(feature):
     # 'OptionContainer',
     # 'PasswordInput',
     "ProgressBar",
+    "ActivityIndicator",
     # 'ScrollContainer',
     # 'Selection',
     # 'Slider',
diff --git a/web/src/toga_web/widgets/activityindicator.py b/web/src/toga_web/widgets/activityindicator.py
new file mode 100644
index 0000000000..9366342146
--- /dev/null
+++ b/web/src/toga_web/widgets/activityindicator.py
@@ -0,0 +1,19 @@
+from .base import Widget
+
+
+class ActivityIndicator(Widget):
+    def create(self):
+        self.native = self._create_native_widget("sl-spinner")
+        self.stop()
+
+    # Actions
+    def start(self):
+        self.native.style.visibility = "visible"
+        self._is_running = True
+
+    def stop(self):
+        self.native.style.visibility = "hidden"
+        self._is_running = False
+
+    def is_running(self):
+        return self._is_running