We have new properties in CSS for styling underlines, as explained here by Jen Simmons.
The relevant properties are:
text-decoration-thicknesstext-decoration-colortext-underline-offsettext-decoration-skip-ink
The skip-ink concept becomes more interesting (to me at least) when you're also using the other properties (especially offset) in order to start the underline higher up the word causing it to appear behind it.