Motivation
The parts of D3 that are meant to directly produce DOM elements are hard to use with frameworks, because manual work is needed to correctly react to changes. Avoiding this is one of the reasons why frontend frameworks exist in the first place, so this feels like a step back for some people.
You can of course use things like d3-axis with frontend frameworks like Preact, but it's not that pretty: you have to use ref callbacks (see Mike Bostock's post for an example). In Vue.js, ref callbacks don't even exist. Many visualization libraries, e.g. vx, implement things like axes from scratch, because it's easier that way.
Web Components allow us to build components that react to changes and are both usable in vanilla JavaScript and modern frameworks.
Usage
This is obviously experimental and not meant to be used in a "serious" environment.
Refer to the LitElement usage guide for a guide on how to use the components.
The API is nearly the same as D3's API: