critical-css-example.css

/* main.css */

/* critical:start */
header{
    background-color: #1d1d1d;
    font-size: 2em;
}

.aside {
    text-decoration: underline;
}

/* critical:end */

p {
    font-size: 14px;
}

li {
    float: left;
    /* critical:start */
    color: red;
    /* critical:end */
}

footer{
    background-color: #1d1d1d;
    font-size: 1.1em;
}

a[rel=external]:before{
    content: '[!]';
    /* critical */
}

/* critical:start */
@media screen and (min-width: 400px) {
    footer {
        padding: 50px;
    }
}
/* critical:end */

@media screen and (min-width: 1400px) {
    header {
        height: 300px;
        /* critical:start */
        background-color: #FF0066;
        font-size: 3em;
        /* critical:end */
    }
}