web-platform-tests · Minju-kim-igalia · Apr 16, 2026 · Apr 16, 2026 · Apr 16, 2026 · Apr 16, 2026
diff --git a/css/cssom-view/devicepixelratio-override-bidi.https.html b/css/cssom-view/devicepixelratio-override-bidi.https.html
@@ -0,0 +1,61 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>devicePixelRatio reflects BiDi setViewport override</title>
+<link rel="help" href="https://www.w3.org/TR/cssom-view/#dom-window-devicepixelratio">
+<link rel="help" href="https://www.w3.org/TR/webdriver-bidi/#command-browsingContext-setViewport">
+<link rel="help" href="https://www.w3.org/TR/mediaqueries-4/#resolution">
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/resources/testdriver.js?feature=bidi"></script>
+<script src="/resources/testdriver-vendor.js"></script>
+
+<script>
+function getTargetDPR() {
+  // Pick a value that's different from the current DPR, so the test
+  // still detects failures even if the default environment is already 2.
+  return window.devicePixelRatio === 2 ? 1 : 2;
+}
+
+function getNonMatchingDPR(target) {
+  return target === 2 ? 1 : 2;
+}
+
+function waitForNextAnimationFrame() {
+  return new Promise(resolve => requestAnimationFrame(resolve));
+}
+
+async function setDevicePixelRatio(value) {
+  await test_driver.bidi.browsing_context.set_viewport({
+    context: window,
+    devicePixelRatio: value,
+  });
+  await waitForNextAnimationFrame();
+}
+
+promise_test(async t => {
+  const targetDPR = getTargetDPR();
+  t.add_cleanup(() => setDevicePixelRatio(null));
+  await setDevicePixelRatio(targetDPR);
+  assert_equals(
+    window.devicePixelRatio,
+    targetDPR,
+    "window.devicePixelRatio should reflect the BiDi override"
+  );
+}, "window.devicePixelRatio reflects a BiDi devicePixelRatio override");
+
+promise_test(async t => {
+  const targetDPR = getTargetDPR();
+  const nonMatchingDPR = getNonMatchingDPR(targetDPR);
+  t.add_cleanup(() => setDevicePixelRatio(null));
+  await setDevicePixelRatio(targetDPR);
+  assert_true(
+    matchMedia(`(resolution: ${targetDPR}dppx)`).matches,
+    "The matching resolution media query should match after the override"
+  );
+  assert_false(
+    matchMedia(`(resolution: ${nonMatchingDPR}dppx)`).matches,
+    "A different resolution media query should not match after the override"
+  );
+}, "resolution media queries reflect a BiDi devicePixelRatio override");
+</script>
@@ -777,6 +777,42 @@
                     },
                 }
             },
+            /**
+             * `browsingContext <https://www.w3.org/TR/webdriver-bidi/#module-browsingContext>`_ module.
+             */
+            browsing_context: {
+                /**
+                 * Sets the viewport and/or device pixel ratio override for the
+                 * specified browsing context. Matches the
+                 * `browsingContext.setViewport
+                 * <https://www.w3.org/TR/webdriver-bidi/#command-browsingContext-setViewport>`_
+                 * WebDriver BiDi command.
+                 *
+                 * @example
+                 * await test_driver.bidi.browsing_context.set_viewport({
+                 *     devicePixelRatio: 2
+                 * });
+                 *
+                 * @param {object} params - Parameters for the command.
+                 * @param {Context} [params.context] The optional browsing
+                 * context to update. If omitted and `params.userContexts` is
+                 * not provided, this wrapper defaults to the current window.
+                 * @param {null|object} [params.viewport] The optional viewport
+                 * override. Passing `null` clears the viewport override.
+                 * @param {null|number} [params.devicePixelRatio] The optional
+                 * device pixel ratio override. Passing `null` clears the DPR
+                 * override.
+                 * @param {Array.<string>} [params.userContexts] The
+                 * optional user contexts to update.
+                 * @returns {Promise<void>} Resolves when the viewport settings
+                 * are successfully applied.
+                 */
+                set_viewport: function(params) {
+                    assertBidiIsEnabled();
+                    return window.test_driver_internal.bidi.browsing_context
+                        .set_viewport(params);
+                },
+            },
             /**
              * `speculation <https://wicg.github.io/nav-speculation/prefetch.html>`_ module.
              */
@@ -2469,6 +2505,12 @@
                     }
                 }
             },
+            browsing_context: {
+                set_viewport: function() {
+                    throw new Error(
+                        'bidi.browsing_context.set_viewport is not implemented by testdriver-vendor.js');
+                },
+            },
             emulation: {
                 set_geolocation_override: function (params) {
                     throw new Error(

diff --git a/tools/wptrunner/wptrunner/executors/asyncactions.py b/tools/wptrunner/wptrunner/executors/asyncactions.py
@@ -163,6 +163,54 @@ async def execute(self, context: str, payload: Mapping[str, Any]) -> Any:
         return await self.protocol.bidi_bluetooth.simulate_descriptor_response(
             context, address, service_uuid, characteristic_uuid, descriptor_uuid, type, code, data)
 
+class BidiBrowsingContextSetViewportAction:
+    name = "bidi.browsing_context.set_viewport"
+
+    def __init__(self, logger, protocol):
+        do_delayed_imports()
+        self.logger = logger
+        self.protocol = protocol
+
+    async def __call__(self, payload):
+        if "context" in payload and "userContexts" in payload:
+            raise ValueError("`context` and `userContexts` are mutually exclusive")
+
+        if "context" in payload:
+            if payload["context"] is None:
+                raise ValueError("`context` must not be null")
+            context = get_browsing_context_id(payload["context"])
+        else:
+            context = None
+
+        viewport = (
+            payload["viewport"]
+            if "viewport" in payload
+            else webdriver.bidi.undefined.UNDEFINED
+        )
+        device_pixel_ratio = (
+            payload["devicePixelRatio"]
+            if "devicePixelRatio" in payload
+            else webdriver.bidi.undefined.UNDEFINED
+        )
+
+        if "userContexts" in payload:
+            user_contexts = payload["userContexts"]
+            if user_contexts is None:
+                raise ValueError("`userContexts` must not be null")
+            if len(user_contexts) == 0:
+                raise ValueError("At least one user context must be provided")
+        else:
+            user_contexts = None
+
+        if context is None and user_contexts is None:
+            raise ValueError("Either `context` or `userContexts` must be provided")
+
+        return await self.protocol.bidi_browsing_context.set_viewport(
+            context=context,
+            viewport=viewport,
+            device_pixel_ratio=device_pixel_ratio,
+            user_contexts=user_contexts)
+
 class BidiEmulationSetGeolocationOverrideAction:
     name = "bidi.emulation.set_geolocation_override"
 
@@ -354,6 +402,7 @@ async def __call__(self, payload):
     BidiBluetoothSimulateCharacteristicResponseAction,
     BidiBluetoothSimulateDescriptorAction,
     BidiBluetoothSimulateDescriptorResponseAction,
+    BidiBrowsingContextSetViewportAction,
     BidiEmulationSetGeolocationOverrideAction,
     BidiEmulationSetLocaleOverrideAction,
     BidiEmulationSetScreenOrientationOverrideAction,

diff --git a/tools/wptrunner/wptrunner/executors/executorwebdriver.py b/tools/wptrunner/wptrunner/executors/executorwebdriver.py
@@ -53,12 +53,15 @@
                        WebExtensionsProtocolPart,
                        merge_dicts)
 
-from typing import Any, List, Dict, Optional
+from typing import Any, List, Dict, Optional, Mapping, TYPE_CHECKING, Union
 from webdriver.client import Session
 from webdriver import error as webdriver_error
 from webdriver.bidi import error as webdriver_bidi_error
 from webdriver.bidi.protocol import bidi_deserialize
 
+if TYPE_CHECKING:
+    from webdriver.bidi.undefined import Undefined
+
 here = os.path.dirname(__file__)
 
 
@@ -278,6 +281,17 @@ async def handle_user_prompt(self,
         await self.webdriver.bidi_session.browsing_context.handle_user_prompt(
             context=context, accept=accept, user_text=user_text)
 
+    async def set_viewport(self,
+                           context: Optional[str],
+                           viewport: Union[Optional[Mapping[str, Any]], "Undefined"],
+                           device_pixel_ratio: Union[Optional[float], "Undefined"],
+                           user_contexts: Optional[List[str]]) -> None:
+        await self.webdriver.bidi_session.browsing_context.set_viewport(
+            context=context,
+            viewport=viewport,
+            device_pixel_ratio=device_pixel_ratio,
+            user_contexts=user_contexts)
+
 
 class WebDriverBidiEventsProtocolPart(BidiEventsProtocolPart):
     _subscriptions: List[str] = []

diff --git a/tools/wptrunner/wptrunner/executors/protocol.py b/tools/wptrunner/wptrunner/executors/protocol.py
@@ -578,6 +578,22 @@ async def handle_user_prompt(self,
         """
         pass
 
+    @abstractmethod
+    async def set_viewport(self,
+                           context: Optional[str],
+                           viewport: Union[Optional[Mapping[str, Any]], "Undefined"],
+                           device_pixel_ratio: Union[Optional[float], "Undefined"],
+                           user_contexts: Optional[List[str]]) -> None:
+        """
+        Modifies specific viewport characteristics for a browsing context or
+        user contexts.
+
+        `viewport=None` or `device_pixel_ratio=None` clears the corresponding
+        override.
+        Passing `UNDEFINED` means the corresponding field is omitted from the
+        BiDi command.
+        """
+        pass
 
 class BidiEventsProtocolPart(ProtocolPart):
     """Protocol part for managing BiDi events"""

diff --git a/tools/wptrunner/wptrunner/testdriver-extra.js b/tools/wptrunner/wptrunner/testdriver-extra.js
@@ -411,6 +411,27 @@
                     'bluetooth.descriptorEventGenerated', on_event);
     };
 
+    window.test_driver_internal.bidi.browsing_context = {
+        set_viewport: function(params) {
+            const actionParams = {...(params ?? {})};
+            const hasContext = "context" in actionParams &&
+                actionParams.context !== undefined;
+            const hasUserContexts = "userContexts" in actionParams &&
+                actionParams.userContexts !== undefined;
+
+            if (hasContext && hasUserContexts) {
+                throw new Error(
+                    "`context` and `userContexts` are mutually exclusive in set_viewport");
+            }
+            if (!hasContext && !hasUserContexts) {
+                // browsingContext.setViewport requires exactly one of `context`
+                // or `userContexts`. Default to the current window if neither was provided.
+                actionParams.context = window;
+            }
+            return create_action("bidi.browsing_context.set_viewport", actionParams);
+        }
+    };
+
     window.test_driver_internal.bidi.emulation.set_geolocation_override =
         function (params) {
             if ('coordinates' in params && 'error' in params) {