CSS:
/* this webkit prefixes are also working in firefox */
background: -webkit-linear-gradient(left, #000, #aaa);
background: linear-gradient(to right, #000, #aaa);
background-clip: text;
-webkit-background-clip: text;
-webkit-text-fill-color: transparent;
background-clip on caniuse.com
-webkit-text-fill-color on caniuse.com