chromium/third_party/blink/web_tests/editing/selection/selection-color.html

<!DOCTYPE html>

<p>This tests that selected text retains its color.</p>

<div id=selectme>
	<p>Text no styling</p>
	<p style="color:red">Text color: red</p>
	<p style="color:red;background-color:gray;">Text color: red, background: gray</p>
	<p class="customSelection">color: red, ::selection background:gray</p>
</div>

<style>
.customSelection {
  color: red;
}
.customSelection::selection {
  background-color: gray;
}
</style>

<script>
    window.getSelection().selectAllChildren(selectme);
</script>