The easiest way to introduce #CSS cascade layers into your existing codebase today:
1. Move your CSS resets into a layer named "reset".
Because the selectors in your reset should already be pretty weak, there should be almost no net effect.
2. Move (some of) your third party styles into a layer named "thirdparty".
Third-party styles should already have lower priority than your own styles, so this again has very little impact.
All the rest of your styles can continue to stay unlayered.