There are many use cases where users expect that their data are end-to-end encrypted and no third party or attacker will be able to read the data except at the user's device. In some cases this is easy to accomplish and give confidence to the user about the end-to-end encryption. Generally, this is because there is a separate app at both ends of the communication and that can be examined with the proper tools.
However, there are two cases where the user wants to be sure that his data are not readable at the server, but cannot rely on a separately installed app.
The first one is data that are accessed via a web browser: