Given the following HTML:

<style>
button {
  color: #fff;
  background: #00633D;
  border-color: #00633D;
  opacity: .65;
}
</style>
<button><span>Hello</span></button>

The foreground color is calculated as #c5dcd3, which is not quite correct. If we put the calculated color next to the rendered HTML, the calculated foreground color is not the same.

I'll need to dig into the color spec again to see how opacity works on text nodes as it appears it should use the background behind the element with opacity instead of blending into all backgrounds.