Burt Totaro, Hodge theory of classifying stacks.
The talk is based on my paper Hodge theory of classifying stacks. The talk combines several topics, so a slight acquaintance with them could be useful.
Quick introductions to several of the topics can be found in my notes on Topological methods in algebraic geometry. This is a bit long (85 pages), but the different chapters can be read independently. Some of the relevant chapters for the talk are "Principal bundles and classifying spaces", "Topology and geometry of Lie groups", and "Stacks".