How slick :is(this CSS!?) 
removes the margins on all headers with a `.tight` class
h1.tight, h2.tight.......
:is(h1,h2).tight {
}

removes the margins on all headers with a `.tight` class
h1.tight, h2.tight.......

:is(h1,h2).tight {

Here's a screenshot from Chrome Canary where it's supported with the chrome://flags/ #enable-experimental-web-platform-features enabled
Codepen https://codepen.io/argyleink/pen/abZvRbV

Codepen https://codepen.io/argyleink/pen/abZvRbV