LayoutTests/webgl/webgl-metal-enabled.html

<canvas id="a"></canvas>
<script>
if (window.testRunner)
    window.testRunner.dumpAsText();

function run()
{
    const out = document.querySelector("p");
    let result = "";

    if (!window.internals) {
        out.innerText = "Test requires internals.";
        return;
    }

    const canvasA = document.getElementById("a");
    const glA = canvasA.getContext("webgl");
    const requested = internals.requestedMetal(glA);
    const supported = internals.platformSupportsMetal(false);
    let resultStr = "OK";
    if (requested != supported)
        resultStr = "FAIL";

    result += `Metal for WebGL1 enabled matches platform support: ${resultStr}<br>`;
    out.innerHTML = result;
}

window.addEventListener("load", run, false);
</script>
<p></p>