matplotlib
diff --git a/doc/release/next_whats_new/pyodide_backend.rst b/doc/release/next_whats_new/pyodide_backend.rst
@@ -0,0 +1,6 @@
+Pyodide backend
+---------------
+
+The ``pyodide`` backend, which has been patched into pyodide builds of Matplotlib since
+early 2025, is now included in the main Matplotlib repository.
+It is an interactive backend based on ``webagg`` and is the default backend used on pyodide.
diff --git a/galleries/users_explain/figure/backends.rst b/galleries/users_explain/figure/backends.rst
@@ -199,6 +199,7 @@ GTK4Cairo Cairo rendering to a GTK_ 4.x canvas (requires PyGObject_ and
           pycairo_).
 wxAgg     Agg rendering to a wxWidgets_ canvas (requires wxPython_ 4).
           This backend can be activated in IPython with ``%matplotlib wx``.
+pyodide   Variant of WebAgg backend that is the default on Pyodide_.
 ========= ================================================================
 
 .. note::
@@ -223,6 +224,7 @@ wxAgg     Agg rendering to a wxWidgets_ canvas (requires wxPython_ 4).
 .. _Tk: https://www.tcl.tk/
 .. _wxWidgets: https://www.wxwidgets.org/
 .. _ipympl: https://www.matplotlib.org/ipympl
+.. _Pyodide: https://pyodide.org/
 
 .. _ipympl_install:
 

diff --git a/lib/matplotlib/backends/backend_pyodide.py b/lib/matplotlib/backends/backend_pyodide.py
@@ -0,0 +1,166 @@
+"""Interactive backend for pyodide running in main browser thread, based on webagg."""
+
+import base64
+from io import BytesIO
+import json
+import mimetypes
+from pathlib import Path
+
+from pyodide.code import run_js
+from pyodide.ffi import create_proxy
+
+from matplotlib.backend_bases import _Backend
+from matplotlib._pylab_helpers import Gcf
+from . import backend_webagg_core as core
+
+
+class FigureManagerPyodide(core.FigureManagerWebAgg):
+    _toolbar2_class = core.NavigationToolbar2WebAgg
+
+    @classmethod
+    def pyplot_show(cls, *, block=None):
+        PyodideApplication.initialize()
+        managers = Gcf.get_all_fig_managers()
+        for manager in managers:
+            manager.show()
+
+    def show(self):
+        fignum = str(self.num)
+
+        js_code = \
+            """
+            var websocket_type = mpl.get_websocket_type();
+            var websocket = new websocket_type(fig_id);
+            const parent_element = document.pyodideMplTarget ?? document.body;
+            const fig = new mpl.figure(fig_id, websocket, null, parent_element);
+            fig;
+            """
+        js_code = f"var fig_id = '{fignum}';" + js_code
+
+        self.js_fig = run_js(js_code)
+        web_socket = PyodideApplication.MockPythonWebSocket(self, self.js_fig.ws)
+        web_socket.open(fignum)
+
+
+class FigureCanvasPyodide(core.FigureCanvasWebAggCore):
+    manager_class = FigureManagerPyodide
+
+    def get_diff_image(self):
+        ret = super().get_diff_image()
+        self._force_full = True
+        return ret
+
+    def handle_save(self, event):
+        figure_id = event['figure_id']
+        format = event['format']
+
+        try:
+            from js import alert, document
+        except ImportError:
+            raise RuntimeError(
+                "Save not supported as cannot import js.alert and js.document")
+
+        mimetype = mimetypes.types_map.get(f".{format}")
+        if mimetype is None:
+            alert(f"Cannot download plot, unable to determine mimetype for '{format}'")
+            return
+
+        element = document.createElement('a')
+        data = BytesIO()
+        self.figure.savefig(data, format=format)
+
+        element.setAttribute(
+            "href",
+            "data:{};base64,{}".format(
+                mimetype, base64.b64encode(data.getvalue()).decode("ascii")
+            ),
+        )
+        element.setAttribute("download", f"plot{figure_id}.{format}")
+        element.style.display = "none"
+        document.body.appendChild(element)
+        element.click()
+        document.body.removeChild(element)
+
+
+class PyodideApplication():
+    initialized = False
+
+    class MockPythonWebSocket:
+        supports_binary = True
+
+        def __init__(self, manager, js_web_socket):
+            self.manager = manager
+            self.js_web_socket = js_web_socket
+            self.on_message_proxy = None
+
+        def open(self, fignum):
+            self.fignum = int(fignum)
+            self.on_message_proxy = create_proxy(self.on_message)
+            self.js_web_socket.open(self.on_message_proxy)
+            self.manager.add_web_socket(self)
+
+        def on_close(self):
+            self.manager.remove_web_socket(self)
+            self.on_message_proxy.destroy()
+            self.on_message_proxy = None
+
+        def on_message(self, message):
+            message = json.loads(message)
+
+            # The 'supports_binary' message is on a client-by-client
+            # basis.  The others affect the (shared) canvas as a
+            # whole.
+            if message['type'] == 'supports_binary':
+                self.supports_binary = message['value']
+            else:
+                # It is possible for a figure to be closed,
+                # but a stale figure UI is still sending messages
+                # from the browser.
+                if self.manager is not None:
+                    self.manager.handle_json(message)
+
+        def send_json(self, content):
+            self.js_web_socket.receive_json(json.dumps(content))
+
+        def send_binary(self, blob):
+            if self.supports_binary:
+                self.js_web_socket.receive_binary(blob, binary=True)
+            else:
+                data_uri = "data:image/png;base64,{}".format(
+                    blob.encode('base64').replace('\n', ''))
+                self.js_web_socket.receive_binary(data_uri)
+
+    @classmethod
+    def initialize(cls, url_prefix='', port=None, address=None):
+        if cls.initialized:
+            return
+
+        try:
+            from js import document
+
+            css = (Path(__file__).parent / "web_backend/css/mpl.css").read_text(
+                encoding="utf-8")
+            style = document.createElement('style')
+            style.textContent = css
+            document.head.append(style)
+        except ImportError:
+            # js.document not available, continue without CSS.
+            pass
+
+        js_content = core.FigureManagerWebAgg.get_javascript(pyodide=True)
+        set_toolbar_image_callback = run_js(js_content)
+        set_toolbar_image_callback(create_proxy(PyodideApplication.get_toolbar_image))
+
+        cls.initialized = True
+
+    @classmethod
+    def get_toolbar_image(cls, image):
+        filename = Path(__file__).parent.parent / f"mpl-data/images/{image}.png"
+        png_bytes = filename.read_bytes()
+        return png_bytes
+
+
+@_Backend.export
+class _BackendPyodide(_Backend):
+    FigureCanvas = FigureCanvasPyodide
+    FigureManager = FigureManagerPyodide
diff --git a/lib/matplotlib/backends/backend_webagg_core.py b/lib/matplotlib/backends/backend_webagg_core.py
@@ -509,14 +509,17 @@ def refresh_all(self):
                     s.send_binary(diff)
 
     @classmethod
-    def get_javascript(cls, stream=None):
+    def get_javascript(cls, stream=None, *, pyodide=False):
         if stream is None:
             output = StringIO()
         else:
             output = stream
 
         output.write((Path(__file__).parent / "web_backend/js/mpl.js")
                      .read_text(encoding="utf-8"))
+        if pyodide:
+            output.write((Path(__file__).parent / "web_backend/js/mpl_pyodide.js")
+                         .read_text(encoding="utf-8"))
 
         toolitems = []
         for name, tooltip, image, method in cls.ToolbarCls.toolitems:
@@ -536,6 +539,11 @@ def get_javascript(cls, stream=None):
         output.write("mpl.default_extension = {};".format(
             json.dumps(FigureCanvasWebAggCore.get_default_filetype())))
 
+        if pyodide:
+            output.write("mpl.toolbar_image_callback = null;\n")
+            output.write("mpl.set_toolbar_image_callback = function(c) {\n")
+            output.write("  mpl.toolbar_image_callback=c;}\n")
+
         if stream is None:
             return output.getvalue()
 

diff --git a/lib/matplotlib/backends/meson.build b/lib/matplotlib/backends/meson.build
@@ -16,6 +16,7 @@ python_sources = [
   'backend_pdf.py',
   'backend_pgf.py',
   'backend_ps.py',
+  'backend_pyodide.py',
   'backend_qt.py',
   'backend_qtagg.py',
   'backend_qtcairo.py',

diff --git a/lib/matplotlib/backends/registry.py b/lib/matplotlib/backends/registry.py
@@ -1,5 +1,6 @@
 from enum import Enum
 import importlib
+import sys
 
 
 class BackendFilter(Enum):
@@ -92,6 +93,9 @@ def __init__(self):
             "notebook": "nbagg",
         }
 
+        if sys.platform == 'emscripten':
+            self._BUILTIN_BACKEND_TO_GUI_FRAMEWORK["pyodide"] = "pyodide"
+
     def _backend_module_name(self, backend):
         if backend.startswith("module://"):
             return backend[9:]

diff --git a/lib/matplotlib/backends/web_backend/.eslintrc.js b/lib/matplotlib/backends/web_backend/.eslintrc.js
diff --git a/lib/matplotlib/backends/web_backend/css/mpl.css b/lib/matplotlib/backends/web_backend/css/mpl.css
@@ -57,7 +57,8 @@
 
 .mpl-widget.active img {
   /* Convert black to tab:blue, approximately */
-  filter: invert(34%) sepia(97%) saturate(468%) hue-rotate(162deg) brightness(96%) contrast(91%);
+  filter: invert(34%) sepia(97%) saturate(468%) hue-rotate(162deg)
+    brightness(96%) contrast(91%);
 }
 
 button.mpl-widget:focus,

diff --git a/lib/matplotlib/backends/web_backend/eslint.config.js b/lib/matplotlib/backends/web_backend/eslint.config.js
@@ -0,0 +1,63 @@
+import js from "@eslint/js";
+import globals from "globals";
+import prettier from "eslint-config-prettier/flat";
+
+export default [
+  {
+    ignores: ["jquery-ui-*/", "node_modules/**"],
+  },
+  js.configs.recommended,
+  {
+    languageOptions: {
+      globals: {
+        ...globals.browser,
+        ...globals.jquery,
+        IPython: "readonly",
+        MozWebSocket: "readonly",
+        mpl: "readonly",
+        mpl_ondownload: "readonly",
+      },
+    },
+    rules: {
+      indent: ["error", 2, { SwitchCase: 1 }],
+      "max-len": ["error", { code: 100 }],
+      "no-unused-vars": [
+        "error",
+        {
+          argsIgnorePattern: "^_",
+          caughtErrors: "none",
+        }
+      ],
+      quotes: ["error", "double", { avoidEscape: true }],
+    },
+  },
+  {
+    files: ["js/**/*.js"],
+    rules: {
+      indent: ["error", 4, { SwitchCase: 1 }],
+      quotes: ["error", "single", { avoidEscape: true }],
+    },
+  },
+  {
+    files: ["js/mpl_pyodide.js"],
+    rules: {
+      "no-unused-vars": [
+        "error",
+        {
+          argsIgnorePattern: "^_",
+          caughtErrors: "none",
+          args: "none",
+        }
+      ],
+    },
+  },
+  prettier,
+  {
+    rules: {
+      "comma-dangle": [
+        "error",
+        { functions: "never", objects: "always-multiline" }
+      ],
+    },
+  }
+];