Brick is for reusable UI elements; unlike Polymer, it doesn't use custom elements to solve non-UI problems.

Brick uses vanilla syntax for the creation of custom elements, where "vanilla" means "as defined in the relevant standards." Brick uses the platform.js polyfill from the polymer project.

From http://brick.readme.io/v2.0/blog/welcome-to-mozilla-brick

    We have very specific goals for Brick- to make building 
    webapp interfaces easier. There are other projects that
    use the same technologies (like Google's excellent
    Polymer) that provide a more complete web application
    development solution. The motto on the Brick project is
    "strong opinions, tightly scoped"- meaning we don't
    intend to solve the problems of code stucture, module
    loading, data binding, or really any other problem other
    than webapp interfaces.

    ... the rising popularity of Google's Polymer and its
    syntax for describing Custom Elements grew, and when
    users came to contribute to Brick and x-tag, they found
    themselves facing an unfamiliar syntax and an underlying
    library that was frequently not well documented. The
    decision was made that if users encounter Custom Element
    registration syntax, it should be the standards-based
    syntax. Brick components are now written in a 'vanilla'
    style, using the standards-based APIs directly.