With firefox-143.0.4-1 and chromium-141.0.7390.65-1, appending a negative value of the amount of the padding appears to work:

table
    {
        border-spacing: 1em 1cap
        padding-bottom: 0;
        padding-top: 0;
        margin-top: -1cap;
        margin-bottom: -1cap;
    }