light intro to the :is() selector syntax and value

is.css

/* before :is() */
button.focus,
button:focus {
  ...
}

/* after :is() */
button:is(.focus, :focus) {
  ...
}


/* before :is() */
article > h1,
article > h2, 
article > h3, 
article > h4, 
article > h5,
article > h6 {
  ...
}

/* after :is() */
article > :is(h1,h2,h3,h4,h5,h6) {
  ...
}

p:is('.best')