Implement

manifest.json

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+{
+	"manifest_version": 2,
+	"name": "Not yet, AV1",
+	"version": "1.0.0",
+	"description": "Extremely lightweight Chrome plugin to disable AV1 on YouTube and other sites.",
+	"icons": {
+		"16": "icons/16.png",
+		"48": "icons/48.png",
+		"128": "icons/128.png"
+	},
+	"content_scripts": [
+		{
+			"matches": [
+				"*://*.youtube.com/*",
+				"*://*.youtube-nocookie.com/*",
+				"*://*.youtu.be/*"
+			],
+			"js": [
+				"src/inject/inject.js",
+				"src/inject/content_script.js"
+			],
+			"run_at": "document_start",
+			"all_frames": true
+		}
+	],
+	"homepage_url": "https://github.com/Shimmermare/NotYetAV1"
+}

src/inject/content_script.js

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+/**
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2015 erkserkserks
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+// This content script runs in an isolated environment and cannot modify any
+// javascript variables on the youtube page. Thus, we have to inject another
+// script into the DOM.
+
+var injectScript = document.createElement('script');
+// Use textContent instead of src to run inject() synchronously
+injectScript.textContent = inject.toString() + "inject();";
+injectScript.onload = function() {
+  // Remove <script> node after injectScript runs.
+  this.parentNode.removeChild(this);
+};
+(document.head || document.documentElement).appendChild(injectScript);
+

src/inject/inject.js

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+/**
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2015 erkserkserks
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+function inject() {
+    override();
+	
+    function override() {
+        // Override video element canPlayType() function
+        var videoElem = document.createElement('video');
+        var origCanPlayType = videoElem.canPlayType.bind(videoElem);
+        videoElem.__proto__.canPlayType = makeModifiedTypeChecker(origCanPlayType);
+        
+        // Override media source extension isTypeSupported() function
+        var mse = window.MediaSource;
+        // Check for MSE support before use
+        if (mse === undefined) return;
+        var origIsTypeSupported = mse.isTypeSupported.bind(mse);
+        mse.isTypeSupported = makeModifiedTypeChecker(origIsTypeSupported);
+    }
+	
+    // return a custom MIME type checker that can defer to the original function
+    function makeModifiedTypeChecker(origChecker) {
+        // Check if a video type is allowed
+        return function (type) {
+            if (type === undefined || type.indexOf('av01') !== -1) return '';
+            // Otherwise, ask the browser
+            return origChecker(type);
+        };
+    }
+}
+