Custom Elements
One of the key features of the Web Components standard is the ability to create custom elements that encapsulate your functionality on an HTML page, rather than having to make do with a long, nested batch of elements that together provide a custom page feature.
More Shadow DOM
An important aspect of web components is encapsulation — being able to keep the markup structure, style, and behavior hidden and separate from other code on the page so that different parts do not clash, and the code can be kept nice and clean. The Shadow DOM API is a key part of this, providing a way to attach a hidden separated DOM to an element.
More HTML templates
When you have to reuse the same markup structures repeatedly on a web page, it makes sense to use some kind of a template rather than repeating the same structure over and over again. This was possible before, but it is made a lot easier by the HTML <template> element (which is well-supported in modern browsers). This element and its contents are not rendered in the DOM, but it can still be referenced using JavaScript.
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