> The fact that you put "contract" in quotes suggests that you know there really is no such thing.

It's in quotes because people seem keen to remind everyone that there's no legal obligation on the part of the browser makers not to break backwards compatibility. The reasoning seems to be that if we can't sue google for a given action, that action must be fine and the people objecting to it must be wrong. I take a rather dim view of this line of reasoning.

> The reason we have as much backwards compatibility as we do is largely historical and always driven by business goals and requirements, as dictated by customers.

As you yourself pointed out, the web is a giant pile of cobbled together technologies that all seemed like a good idea at the time. If breaking changes were an option, there is a _long_ list of potential depreciation to pick from which would greatly simplify development of both browsers and websites/apps. Further, new features/standards would be able to be added with much less care, since if problems were found in those standards they could be removed/reworked. Despite those huge benefits, no such changes are/should be made, because the costs breaking backwards compatibility are just that high. Maintaining the implied promise that software written for the web will continue to work is a business requirement, because it's crucial for the long term health of the ecosystem.