<!DOCTYPE html>
<style>
textarea {
    font-size: 3em;
    height: 300px;
    outline: none;
}
::selection {
    stroke-color: rgba(63, 128, 33, 0.95); /* green; alpha < 1 so that we don't blend the background color with white. */
    stroke-width: 2px;
}
</style>
<textarea>Any textual selection in this sentence should have a green stroke color.</textarea>
<script>
var textarea = document.querySelector("textarea");
textarea.select();
if (window.testRunner)
    testRunner.setWindowIsKey(false);
</script>