Back when we looked at Odyssey block’s online content we had to go through quite a few sites, but over time most of those get dropped. Mirrodin didn’t have its own website like Invasion and Odyssey, and the last update on the Magic Multiverse page was just link to a description of the Mirrodin novel, nothing more. But the Magic Book Archive is still going, and Magicthegathering.com has a few feature articles and arcana posts that deal with the story or flavor.
Let’s start with the feature articles on Magicthegathering.com, as I assume those would be the most widely read source from back in the day. There is one for each of the three sets and the serve mostly as a quick introduction to the setting and to plug the books. Unfortunately for the continuity obsessives there are a few inconsistencies between these articles and the books they are trying to sell.