::part and ::theme, an ::explainer

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Monica Dinculescu on ::part and ::theme, two pseudo-elements that are very likely to gain traction and receive more attention in the new year. They're designed to help us create and style web components, as Monica explains:

The current new proposal is ::part and ::theme, a set of pseudo-elements that allow you to style inside a shadow tree, from outside of that shadow tree. Unlike :shadow and /deep/, they don’t allow you to style arbitrary elements inside a shadow tree: they only allow you to style elements that an author has tagged as being eligible for styling. They’ve already gone through the CSS working group and were blessed, and were brought up at TPAC at a Web Components session, so we’re confident they’re both the right approach and highly likely to be implemented as a spec by all browsers.

If the whole "shadow tree" phrase makes you panic as much as me then not to worry! Monica has already written an excellent piece that goes into great depth about web components and the Shadow DOM spec, too.