Building a web app can be difficult if it must work with lots and lots of browsers, especially old versions that don't support modern features. The Baseline concept establishes stable web features, and is a good starting point for what is considered a supported feature of the web, browsers, and HTTP.

We want to build web apps using modern features, since they exist to solve problems and provide better solutions for users. We don't want to have to always be shimming or supporting older browsers that a lot of users aren't actually using.

We don't want to support older browsers and older baselines. This would essentially establish our own baseline to maintain and analyze, and Mozilla is better at that.

When features are promoting to Baseline, developers will find them available in Brut soonafter and expect to be able to build web apps using all the modern features available in the majority of browsers in use.

Some users may be left behind by features not supported by browsers they are forced to use. Brut will likely not prevent developers from handling these browsers, but it's not going to provide explicit support.

There has to be a lower-case b baseline, and it might as well be something the community is organizing around