I inherited a similar beast at my last startup. The company ended up going under before I could fully untangle the mess, but when I was researching, I found little out there to give me good advice. All of our HTML, CSS, and JS needed to be addressed and we were on a 2-week release cycle, so I found breaking this debt work into release-size chunks to be a real challenge and still am without a real solution for.

I'd be interested in hearing your approaches to your current project and would definitely be willing to give whatever advice I can based on what I found through trial and error.