But, Widgets!
We started showing non-styled non-widgets because it's important to realize that custom elements are a primitive. The idea is not to lock users into a framework, but the opposite, to unleash creativity.
But it's not fun to start at the bottom, so the Polymer team is working on a nicely styled widget set to blaze the trail.
Click the drawer button in the upper-left corner.
Notes for the advanced:
The particular custom elements used here are not yet canonical, they are just examples. A key takeaway is that the underlying implementations are easily modified without necessarily impacting the app itself.
For example, these layout tools use absolute positioning. Soon, we will have superior objects that use, for example, flex-box or CSS grids. Those improved elements can just drop in.