With the ::spelling-error pseudo introduced in css-pseudo-4, if you want for example to highlight spelling mistakes with a green wavy underline, you can do this:

::spelling-error {
  text-decoration: wavy green underline;
}

However, this only works if the browser's original spelling mistake marker was created using the same mechanism in the UA stylesheet. If it is created some other way, what you'd get instead would be this:

Since that's not what the author would want, I think we should normatively require that browsers that support ::spelling-error and ::grammar-error must create the error indicators using these pseudos in the UA stylesheet.