The browser connection limits per domain weren't baked into any standards, so it's something that has been dramatically improved over time. Most browsers maxed out at two connections five years ago, now six is the most common limit.
Edit: turns out the limit of two connections is in the RFC for http 1.1 but browsers starting with ie 8 have ignore that part of the spec.
>Clients that use persistent connections SHOULD limit the number of simultaneous connections that they maintain to a given server. A single-user client SHOULD NOT maintain more than 2 connections with any server or proxy.
Edit: turns out the limit of two connections is in the RFC for http 1.1 but browsers starting with ie 8 have ignore that part of the spec.