LayoutTests/fast/canvas/webgl/context-creation-attributes.html

<script>
if (window.testRunner)
    window.testRunner.dumpAsText();

function describe(object) {
    result = {};
    if (typeof object === "function")
        object = object.prototype;
    for (let o = object; o; o = o.__proto__) {
        for (let name of Object.getOwnPropertyNames(o)) {
            result[name] = object[name];
        }
    }
    return result;
}

function testContextCreation(parameters)
{
    var canvas = document.createElement("canvas");
    var gl = canvas.getContext("webgl", parameters || {});
    var attributes = gl.getContextAttributes();
    var result = document.createElement("pre");
    result.textContent = JSON.stringify(describe(attributes));
    document.body.appendChild(result);
}

function run() {
    testContextCreation({});
    testContextCreation({stencil: true});
    testContextCreation({powerPreference: "default"});
    testContextCreation({failIfMajorPerformanceCaveat: true});
}

window.addEventListener("load", run, false);
</script>
<p>
    Note that some of the values tested here might not be supported, and thus should return the default values.
</p>