diff --git a/build.sbt b/build.sbt
index 220ac30514..f1711095da 100644
--- a/build.sbt
+++ b/build.sbt
@@ -18,6 +18,7 @@ import java.io.File
 
 import com.typesafe.tools.mima.core._
 import org.openqa.selenium.firefox.FirefoxOptions
+import org.openqa.selenium.firefox.FirefoxProfile
 import org.scalajs.jsenv.selenium.SeleniumJSEnv
 
 ThisBuild / baseVersion := "3.1"
@@ -70,7 +71,8 @@ ThisBuild / githubWorkflowBuild := Seq(
   WorkflowStep.Sbt(List("${{ matrix.ci }}")),
   WorkflowStep.Sbt(
     List("docs/mdoc"),
-    cond = Some(s"(matrix.scala == '$Scala213' || matrix.scala == '$Scala3') && matrix.ci == 'ciJVM'")),
+    cond = Some(
+      s"(matrix.scala == '$Scala213' || matrix.scala == '$Scala3') && matrix.ci == 'ciJVM'")),
   WorkflowStep.Sbt(
     List("exampleJVM/compile"),
     cond = Some(s"matrix.ci == 'ciJVM' && matrix.os == '$PrimaryOS'")),
@@ -128,7 +130,10 @@ ThisBuild / Test / jsEnv := {
   val old = (Test / jsEnv).value
 
   if (useFirefoxEnv.value) {
+    val profile = new FirefoxProfile()
+    profile.setPreference("privacy.file_unique_origin", false)
     val options = new FirefoxOptions()
+    options.setProfile(profile)
     options.addArguments("-headless")
     new SeleniumJSEnv(options)
   } else {
@@ -166,13 +171,22 @@ addCommandAlias("ciJS", "; project rootJS; headerCheck; scalafmtCheck; clean; te
 // we do the firefox ci *only* on core because we're only really interested in IO here
 addCommandAlias(
   "ciFirefox",
-  "; set Global / useFirefoxEnv := true; project rootJS; headerCheck; scalafmtCheck; clean; testsJS/test; set Global / useFirefoxEnv := false"
+  "; set Global / useFirefoxEnv := true; project rootJS; headerCheck; scalafmtCheck; clean; testsJS/test; webWorkerTests/test; set Global / useFirefoxEnv := false"
 )
 
 addCommandAlias("prePR", "; root/clean; +root/scalafmtAll; +root/headerCreate")
 
 val jsProjects: Seq[ProjectReference] =
-  Seq(kernel.js, kernelTestkit.js, laws.js, core.js, testkit.js, tests.js, std.js, example.js)
+  Seq(
+    kernel.js,
+    kernelTestkit.js,
+    laws.js,
+    core.js,
+    testkit.js,
+    tests.js,
+    webWorkerTests,
+    std.js,
+    example.js)
 
 val undocumentedRefs =
   jsProjects ++ Seq[ProjectReference](benchmarks, example.jvm)
@@ -222,14 +236,14 @@ lazy val kernel = crossProject(JSPlatform, JVMPlatform)
     name := "cats-effect-kernel",
     libraryDependencies ++= Seq(
       ("org.specs2" %%% "specs2-core" % Specs2Version % Test).cross(CrossVersion.for3Use2_13),
-      "org.typelevel" %%% "cats-core" % CatsVersion))
-  .jsSettings(
-    Compile / doc / sources := {
-      if (isDotty.value)
-        Seq()
-      else
-        (Compile / doc / sources).value
-    })
+      "org.typelevel" %%% "cats-core" % CatsVersion)
+  )
+  .jsSettings(Compile / doc / sources := {
+    if (isDotty.value)
+      Seq()
+    else
+      (Compile / doc / sources).value
+  })
 
 /**
  * Reference implementations (including a pure ConcurrentBracket), generic ScalaCheck
@@ -278,16 +292,21 @@ lazy val core = crossProject(JSPlatform, JVMPlatform)
       ProblemFilters.exclude[MissingClassProblem]("cats.effect.AsyncPropagateCancelation$"),
       // introduced by #1913, striped fiber callback hashtable, changes to package private code
       ProblemFilters.exclude[MissingClassProblem]("cats.effect.unsafe.FiberErrorHashtable"),
-      ProblemFilters.exclude[IncompatibleResultTypeProblem]("cats.effect.unsafe.IORuntime.fiberErrorCbs"),
+      ProblemFilters.exclude[IncompatibleResultTypeProblem](
+        "cats.effect.unsafe.IORuntime.fiberErrorCbs"),
       ProblemFilters.exclude[IncompatibleMethTypeProblem]("cats.effect.unsafe.IORuntime.this"),
-      ProblemFilters.exclude[IncompatibleResultTypeProblem]("cats.effect.unsafe.IORuntime.<init>$default$6"),
+      ProblemFilters.exclude[IncompatibleResultTypeProblem](
+        "cats.effect.unsafe.IORuntime.<init>$default$6"),
       // introduced by #1928, wake up a worker thread before spawning a helper thread when blocking
       // changes to `cats.effect.unsafe` package private code
-      ProblemFilters.exclude[IncompatibleResultTypeProblem]("cats.effect.unsafe.WorkStealingThreadPool.notifyParked"),
+      ProblemFilters.exclude[IncompatibleResultTypeProblem](
+        "cats.effect.unsafe.WorkStealingThreadPool.notifyParked"),
       // introduced by #2041, Rewrite and improve `ThreadSafeHashtable`
       // changes to `cats.effect.unsafe` package private code
-      ProblemFilters.exclude[DirectMissingMethodProblem]("cats.effect.unsafe.ThreadSafeHashtable.hashtable"),
-      ProblemFilters.exclude[DirectMissingMethodProblem]("cats.effect.unsafe.ThreadSafeHashtable.hashtable_="),
+      ProblemFilters.exclude[DirectMissingMethodProblem](
+        "cats.effect.unsafe.ThreadSafeHashtable.hashtable"),
+      ProblemFilters.exclude[DirectMissingMethodProblem](
+        "cats.effect.unsafe.ThreadSafeHashtable.hashtable_="),
       // introduced by #2051, Tracing
       // changes to package private code
       ProblemFilters.exclude[DirectMissingMethodProblem]("cats.effect.IO#Blocking.apply"),
@@ -299,7 +318,8 @@ lazy val core = crossProject(JSPlatform, JVMPlatform)
       ProblemFilters.exclude[DirectMissingMethodProblem]("cats.effect.IO#FlatMap.apply"),
       ProblemFilters.exclude[DirectMissingMethodProblem]("cats.effect.IO#FlatMap.copy"),
       ProblemFilters.exclude[DirectMissingMethodProblem]("cats.effect.IO#FlatMap.this"),
-      ProblemFilters.exclude[DirectMissingMethodProblem]("cats.effect.IO#HandleErrorWith.apply"),
+      ProblemFilters.exclude[DirectMissingMethodProblem](
+        "cats.effect.IO#HandleErrorWith.apply"),
       ProblemFilters.exclude[DirectMissingMethodProblem]("cats.effect.IO#HandleErrorWith.copy"),
       ProblemFilters.exclude[DirectMissingMethodProblem]("cats.effect.IO#HandleErrorWith.this"),
       ProblemFilters.exclude[DirectMissingMethodProblem]("cats.effect.IO#Map.apply"),
@@ -312,7 +332,9 @@ lazy val core = crossProject(JSPlatform, JVMPlatform)
       ProblemFilters.exclude[MissingClassProblem]("cats.effect.SyncIO$Delay"),
       ProblemFilters.exclude[DirectMissingMethodProblem]("cats.effect.IO#IOCont.apply"),
       ProblemFilters.exclude[DirectMissingMethodProblem]("cats.effect.IO#IOCont.copy"),
-      ProblemFilters.exclude[DirectMissingMethodProblem]("cats.effect.IO#IOCont.this")))
+      ProblemFilters.exclude[DirectMissingMethodProblem]("cats.effect.IO#IOCont.this")
+    )
+  )
   .jvmSettings(
     javacOptions ++= Seq("-source", "1.8", "-target", "1.8")
   )
@@ -345,6 +367,20 @@ lazy val tests = crossProject(JSPlatform, JVMPlatform)
     Test / fork := true,
     Test / javaOptions += s"-Dsbt.classpath=${(Test / fullClasspath).value.map(_.data.getAbsolutePath).mkString(File.pathSeparator)}")
 
+lazy val webWorkerTests = project
+  .in(file("webworker-tests"))
+  .dependsOn(tests.js % "compile->test")
+  .enablePlugins(ScalaJSPlugin, BuildInfoPlugin, NoPublishPlugin)
+  .settings(
+    name := "cats-effect-webworker-tests",
+    scalaJSUseMainModuleInitializer := true,
+    libraryDependencies += ("org.scala-js" %%% "scalajs-dom" % "1.1.0")
+      .cross(CrossVersion.for3Use2_13),
+    (Test / test) := (Test / test).dependsOn(Compile / fastOptJS).value,
+    buildInfoKeys := Seq[BuildInfoKey](scalaVersion, baseDirectory),
+    buildInfoPackage := "cats.effect"
+  )
+
 /**
  * Implementations lof standard functionality (e.g. Semaphore, Console, Queue)
  * purely in terms of the typeclasses, with no dependency on IO. In most cases,
@@ -388,7 +424,4 @@ lazy val benchmarks = project
   .settings(name := "cats-effect-benchmarks")
   .enablePlugins(NoPublishPlugin, JmhPlugin)
 
-lazy val docs = project
-  .in(file("site-docs"))
-  .dependsOn(core.jvm)
-  .enablePlugins(MdocPlugin)
+lazy val docs = project.in(file("site-docs")).dependsOn(core.jvm).enablePlugins(MdocPlugin)
diff --git a/core/js/src/main/scala/cats/effect/unsafe/PolyfillExecutionContext.scala b/core/js/src/main/scala/cats/effect/unsafe/PolyfillExecutionContext.scala
index e9bc9e3d74..816d850273 100644
--- a/core/js/src/main/scala/cats/effect/unsafe/PolyfillExecutionContext.scala
+++ b/core/js/src/main/scala/cats/effect/unsafe/PolyfillExecutionContext.scala
@@ -113,6 +113,21 @@ private[unsafe] object PolyfillExecutionContext extends ExecutionContext {
           js.Dynamic.global.postMessage(messagePrefix + handle, "*")
           ()
         }
+      } else if (js.typeOf(js.Dynamic.global.MessageChannel) != Undefined) {
+        val channel = js.Dynamic.newInstance(js.Dynamic.global.MessageChannel)()
+
+        channel.port1.onmessage = { event: js.Dynamic =>
+          runIfPresent(event.data.asInstanceOf[Int])
+        }
+
+        { k =>
+          val handle = nextHandle
+          nextHandle += 1
+
+          tasksByHandle += (handle -> k)
+          channel.port2.postMessage(handle)
+          ()
+        }
       } else {
         // we don't try to look for process.nextTick since scalajs doesn't support old node
         // we're also not going to bother fast-pathing for IE6; just fall through
diff --git a/project/plugins.sbt b/project/plugins.sbt
index 26fa3c8185..97417c72cb 100644
--- a/project/plugins.sbt
+++ b/project/plugins.sbt
@@ -7,3 +7,4 @@ addSbtPlugin("pl.project13.scala" % "sbt-jmh" % "0.4.3")
 addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.4.3")
 addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.2.22")
 addSbtPlugin("com.eed3si9n" % "sbt-unidoc" % "0.4.3")
+addSbtPlugin("com.eed3si9n" % "sbt-buildinfo" % "0.10.0")
diff --git a/webworker-tests/src/main/scala/cats/effect/IOSpecRunner.scala b/webworker-tests/src/main/scala/cats/effect/IOSpecRunner.scala
new file mode 100644
index 0000000000..4a187f7f7f
--- /dev/null
+++ b/webworker-tests/src/main/scala/cats/effect/IOSpecRunner.scala
@@ -0,0 +1,53 @@
+/*
+ * Copyright 2020-2021 Typelevel
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package cats.effect
+
+import org.scalajs.dom.webworkers.DedicatedWorkerGlobalScope
+import org.specs2.control.ExecuteActions
+import org.specs2.reporter.BufferedLineLogger
+import org.specs2.runner.ClassRunner
+import org.specs2.runner.Runner
+import org.specs2.specification.core.Env
+
+import scala.scalajs.js
+
+object IOSpecRunner extends IOApp.Simple with ClassRunner {
+
+  def postMessage(msg: js.Any): Unit = DedicatedWorkerGlobalScope.self.postMessage(msg)
+
+  override def run: IO[Unit] = IO.fromFuture {
+    IO {
+      val spec = new IOSpec
+      val env = Env(lineLogger = new BufferedLineLogger {
+        override def infoLine(msg: String): Unit = postMessage(s"[info] $msg")
+        override def failureLine(msg: String): Unit = postMessage(s"[error] $msg")
+        override def errorLine(msg: String): Unit = postMessage(s"[error] $msg")
+        override def warnLine(msg: String): Unit = postMessage(s"[warn] $msg")
+      })
+      val loader = new ClassLoader() {}
+      val action = for {
+        printers <- createPrinters(env.arguments, loader).toAction
+        stats <- Runner.runSpecStructure(spec.structure(env), env, loader, printers)
+        // TODO I have no idea how to suspend effects in this
+        _ = postMessage(stats.toString)
+        _ = postMessage(stats.isSuccess)
+      } yield ()
+      ExecuteActions.runActionFuture(action)(env.executionEnv)
+    }
+  }
+
+}
diff --git a/webworker-tests/src/test/scala/cats/effect/WebWorkerIOSpec.scala b/webworker-tests/src/test/scala/cats/effect/WebWorkerIOSpec.scala
new file mode 100644
index 0000000000..a3778b98a9
--- /dev/null
+++ b/webworker-tests/src/test/scala/cats/effect/WebWorkerIOSpec.scala
@@ -0,0 +1,56 @@
+/*
+ * Copyright 2020-2021 Typelevel
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package cats.effect
+
+import org.scalajs.dom.webworkers.Worker
+import org.scalajs.dom.window
+
+import scala.concurrent.duration._
+import scala.util.Try
+
+class WebWorkerIOSpec extends BaseSpec {
+
+  override def executionTimeout = 5.minutes // This is to run the _entire_ IOSpec
+
+  def scalaVersion = if (BuildInfo.scalaVersion.startsWith("2"))
+    BuildInfo.scalaVersion.split("\\.").init.mkString(".")
+  else
+    BuildInfo.scalaVersion
+
+  def targetDir = s"${BuildInfo.baseDirectory}/target/scala-${scalaVersion}"
+
+  Try(window).toOption.foreach { _ =>
+    "io on webworker" should {
+      "pass the spec" in real {
+        for {
+          worker <- IO(
+            new Worker(s"file://${targetDir}/cats-effect-webworker-tests-fastopt/main.js"))
+          success <- IO.async_[Boolean] { cb =>
+            worker.onmessage = { event =>
+              event.data match {
+                case log: String => println(log)
+                case success: Boolean => cb(Right(success))
+                case _ => ()
+              }
+            }
+          }
+        } yield success mustEqual true
+      }
+    }
+  }
+
+}
diff --git a/webworker-tests/src/test/scala/java/io/File.scala b/webworker-tests/src/test/scala/java/io/File.scala
new file mode 100644
index 0000000000..b03c6cc7b3
--- /dev/null
+++ b/webworker-tests/src/test/scala/java/io/File.scala
@@ -0,0 +1,22 @@
+/*
+ * Copyright 2020-2021 Typelevel
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package java.io
+
+// hack hack buildinfo hack
+class File(path: String) {
+  override def toString() = path
+}