<!DOCTYPE html>
<style>
textarea {
    font-size: 3em;
    height: 300px;
    text-shadow: 1px 1px 2px rgba(63, 128, 33, 0.95); /* green; alpha < 1 so that we don't blend the background color with white. */
}
</style>
<textarea>Any textual selection in this sentence should have a green shadow.</textarea>