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diff --git a/dom/canvas/test/webgl-conf/checkout/conformance2/rendering/rasterizer-discard-and-implicit-clear.html b/dom/canvas/test/webgl-conf/checkout/conformance2/rendering/rasterizer-discard-and-implicit-clear.html
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+++ b/dom/canvas/test/webgl-conf/checkout/conformance2/rendering/rasterizer-discard-and-implicit-clear.html
@@ -0,0 +1,149 @@
+<!--
+Copyright (c) 2020 The Khronos Group Inc.
+Use of this source code is governed by an MIT-style license that can be
+found in the LICENSE.txt file.
+-->
+
+<!DOCTYPE html>
+<html>
+<head>
+<meta charset="utf-8">
+<title>RASTERIZER_DISCARD doesn't affect implicit clears</title>
+<link rel="stylesheet" href="../../resources/js-test-style.css"/>
+<script src="../../js/js-test-pre.js"></script>
+<script src="../../js/webgl-test-utils.js"></script>
+<script id="vshader" type="x-shader/x-vertex">#version 300 es
+layout(location=0) in vec2 vPosition;
+uniform float xTranslation;
+void main(void) {
+  gl_Position = vec4(vPosition[0] + xTranslation, vPosition[1], 0.0, 1.0);
+}
+</script>
+<script id="fshader" type="x-shader/x-fragment">#version 300 es
+precision mediump float;
+uniform vec4 color;
+out vec4 outColor;
+void main() {
+  outColor = color;
+}
+</script>
+</head>
+<body>
+<canvas id="example"></canvas>
+<div id="description"></div>
+<div id="console"></div>
+<script>
+"use strict";
+debug("");
+
+description("Enabling RASTERIZER_DISCARD should not affect implicit clears");
+
+const wtu = WebGLTestUtils;
+const canvas = document.getElementById("example");
+const sz = canvas.width = canvas.height = 256;
+const gl = wtu.create3DContext(canvas, undefined, 2);
+const NUM_FRAMES = 15;
+let framesToGo = NUM_FRAMES;
+let xTranslationLoc;
+let colorLoc;
+const positionLocation = 0;
+const red = [ 1.0, 0.0, 0.0, 1.0 ];
+const green = [ 0.0, 1.0, 0.0, 1.0 ];
+const transparentBlackRender = [ 0, 0, 0, 0 ];
+const greenRender = [ 0, 255, 0, 255 ];
+
+if (!gl) {
+    testFailed("WebGL context creation failed");
+    finishTest();
+} else {
+    testPassed("WebGL context creation succeeded");
+    runDrawTest();
+}
+
+function runDrawTest() {
+    debug("Verify that draws with rasterizer discard enabled do not interfere with implicit clears");
+    let prog = wtu.loadProgramFromScript(gl, "vshader", "fshader");
+    gl.useProgram(prog);
+    xTranslationLoc = gl.getUniformLocation(prog, "xTranslation");
+    colorLoc = gl.getUniformLocation(prog, "color");
+    let leftRectBuffer = gl.createBuffer();
+    gl.enableVertexAttribArray(positionLocation);
+    gl.bindBuffer(gl.ARRAY_BUFFER, leftRectBuffer);
+    // Create a rectangle covering the left half of the viewport, in
+    // normalized device coordinates.
+    gl.bufferData(gl.ARRAY_BUFFER, new Float32Array([
+         0.0,  1.0,
+        -1.0,  1.0,
+        -1.0, -1.0,
+         0.0,  1.0,
+        -1.0, -1.0,
+         0.0, -1.0]), gl.STATIC_DRAW);
+    gl.vertexAttribPointer(positionLocation, 2, gl.FLOAT, false, 0, 0);
+    requestAnimationFrame(renderDrawTestFrame);
+}
+
+function renderDrawTestFrame() {
+    // Animation is required in order to expose this bug. When it
+    // occurs, the rectangle leaves trails behind it.
+    gl.uniform1f(xTranslationLoc, 0.0);
+    gl.enable(gl.RASTERIZER_DISCARD);
+    gl.uniform4fv(colorLoc, red);
+    gl.drawArrays(gl.TRIANGLES, 0, 6);
+    gl.disable(gl.RASTERIZER_DISCARD);
+
+    // Animate the rectangle from the left to the right half of the viewport.
+    gl.uniform1f(xTranslationLoc, (NUM_FRAMES - framesToGo) / NUM_FRAMES);
+    // Draw the last frame with green so any (incorrect) trails are visibly red.
+    if (framesToGo == 0)
+        gl.uniform4fv(colorLoc, green);
+    gl.drawArrays(gl.TRIANGLES, 0, 6);
+
+    if (framesToGo-- == 0) {
+        // The left half of the canvas should be transparent black,
+        // which comes from the implicit clear just before drawing the
+        // rectangle without rasterizer discard enabled.
+        wtu.checkCanvasRect(gl, 0, 0, sz / 2, sz, transparentBlackRender, "left half of canvas should be clear", 3);
+        // The right half of the canvas should be solid green, from
+        // the last render of the translated rectangle.
+        wtu.checkCanvasRect(gl, sz / 2, 0, sz / 2, sz, greenRender, "right half of canvas should be green", 3);
+        runReadPixelsTest();
+    } else {
+        requestAnimationFrame(renderDrawTestFrame);
+    }
+}
+
+function runReadPixelsTest() {
+    debug("Verify that readPixels with rasterizer discard enabled receives implicitly cleared data");
+    framesToGo = NUM_FRAMES;  // Reset state.
+    // Clear to transparent black.
+    gl.clearColor(0.0, 0.0, 0.0, 0.0);
+    gl.clear(gl.COLOR_BUFFER_BIT);
+    // Start with rasterizer discard enabled.
+    gl.enable(gl.RASTERIZER_DISCARD);
+    requestAnimationFrame(renderReadPixelsTestFrame);
+}
+
+function renderReadPixelsTestFrame() {
+    // Rasterizer discard is enabled at the beginning of this test.
+
+    // The canvas should always contain transparent black at the beginning of the frame.
+    wtu.checkCanvasRect(gl, 0, 0, sz, sz, transparentBlackRender, undefined, 3);
+
+    gl.disable(gl.RASTERIZER_DISCARD);
+    // Clear to red.
+    gl.clearColor(1.0, 0.0, 0.0, 1.0);
+    gl.clear(gl.COLOR_BUFFER_BIT);
+    // Enable rasterizer discard again.
+    gl.enable(gl.RASTERIZER_DISCARD);
+
+    if (--framesToGo == 0) {
+        finishTest();
+    } else {
+        requestAnimationFrame(renderReadPixelsTestFrame);
+    }
+}
+
+</script>
+
+</body>
+</html>