How do I animate a CSS gradient stop with a smooth transition to transparent?

I have the code bellow. How do I transition the gradient stop smoothly? Its just abruptly changing from one to another.

Most examples of gradient animations I have seem use the gradient position, but I believe changing the gradient stop should be possible too.

.test {
    display: inline-block;
    width: 300px;
    height: 300px;
    border-radius: 100%;
    background: conic-gradient(red, red);
    background-repeat: no-repeat;
    background-size: cover;
    background-position: center center;
    background-size: auto;
    -webkit-mask:radial-gradient(circle, transparent 50%, white calc(50% + 1px));

    animation:
        rotate
        4.5s
        ease-out
        0s
        infinite
        none
        running;
}

@keyframes rotate {
    0% {
        background-image: conic-gradient(red, red);
    }

    30% {
        background-image: conic-gradient(red 70%, transparent);
    }

    70% {
        background-image: conic-gradient(red 30%, transparent, transparent, transparent);
    }

    100% {
        background-image: conic-gradient(red, transparent);
    }
}

<div class="test"></div>

Solution

This as of Dec 03 2020 only works on Chrome or Edge 95+

One can animate the gradient using CSS @property.

@property --opacity {
  syntax: '<percentage>';
  initial-value: 100%;
  inherits: false;
}

.test {
    display: inline-block;
    position: absolute;
    border-radius: 100%;
    background-image: conic-gradient(
        red var(--opacity),
        red 10%,
        rgba(255, 0, 0, var(--opacity)),
        transparent,
        transparent
    );
    will-change: transform, background-image;
    width: 200px;
    height: 200px;
    mask:radial-gradient(circle, transparent 47%, white calc(47% + 1px));
    -webkit-mask:radial-gradient(circle, transparent 47%, white calc(47% + 1px));

    animation:
        conic-gradient
            4.5s
            ease-out
            0s
            infinite
            none
            running;
}

@keyframes conic-gradient {
    50% {
        --opacity: 0%;
    }

    85% {
        --opacity: 100%;
    }
}

<div class="test"></div>