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Developing Web Components by Jason Strimpel, Jarrod Overson

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Chapter 11. Working with the Shadow DOM

The shadow DOM is not the dark side of the DOM, but if it were I would definitely give in to my hatred of the lack of encapsulation the DOM normally affords and cross over.

One of the aspects of the DOM that makes development of widgets/components difficult is this lack of encapsulation. For instance, one major problem has always been CSS rules bleeding into or out of a component’s branch of the DOM tree: it forces one to write overly specific selectors or abuse !important so that styles do not conflict, and even then conflicts still happen in large applications. Another issue caused by lack of encapsulation is that code external to a component can still traverse into the component’s branch of the DOM tree. These problems and others can be prevented by using the shadow DOM.

What Is the Shadow DOM?

So what exactly is this mysterious-sounding shadow DOM? According to the W3C:

Shadow DOM is an adjunct tree of DOM nodes. These shadow DOM subtrees can be associated with an element, but do not appear as child nodes of the element. Instead the subtrees form their own scope. For example, a shadow DOM subtree can contain IDs and styles that overlap with IDs and styles in the document, but because the shadow DOM subtree (unlike the child node list) is separate from the document, the IDs and styles in the shadow DOM subtree do not clash with those in the document.

Note

I am an admirer of the W3C, but oftentimes their specifications, ...

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