Pseudo element content alt text support OR custom transform help

[jossmac]

How would you implement a custom transform for content alternative text?

/* input */
.class-name::after {
  content: "chevron_right" / "";
}

/* desired output */
.class-name::after {
  content: "chevron_right" / "";
}
@supports not (content: "x" / "y") {
  .class-name::after {
    alt: "";
    content: "chevron_right";
  }
}

Ideally this would be built into the library and respond to the targets option. Failing that, I'd really appreciate help implementing a robust custom transform.