Not on line but the RYA booklet G2 is excellent. The full regulations with explanatory footnotes. IMHO more convenient than on an line source as it can be slipped into a pocket/handbag and studied at any convenient moments. Eg during boring meetings, on trains etc etc
Another book is the Seamans Guide to the Rules of the Road, a lot of my students say that it is excellent for a beginner and also for the "expert" to learn the rules.
And lastly the Flip cards, they are excellent for learning with but the only cover certain scenarios and don't do the full rules, but for a way of learning most of the rules they are good.
Seaman's guide is an excellent aid as well as the flip cards. Just be careful about "plain english" versions as it may lead to an interpretation that is at odds with the actual rule.
The flash cards aren't enough by themselves. I examined someone a while ago who was very ropy on the IRPCS. He protested that he knew the flash-cards off by heart. His problem was that I didn't test him on the cards, but described scenarios and asked him what he would do - in other words simple situations in which the rules have to be applied.
"But I know the cards off by heart" doesn't help you apply the rules in practice!
I say again - John Campbell's 'Yachtman's Guide to the Collision Rules', from Adlard Coles and others. That's the best I've come across, and my students agree. It still takes time on the water, though, and experience in identifying not only what presents itself, but what is developing.......
I haven't found an online or e-trainer product that's quite good enough.