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1) Declarative 2) Someone could _finally_ implement something like <grid> to be used instead of hacky <div>s or old-school <table>s to get a proper grid layout _quickly_ as it should be. Something that you get in WPF/Silverlight for free.

Encapsulation: the HTML, JS and CSS that comprise a web component are encapsulated as and within said component.

See also Google Polymer and its <polymer-element> "sugar" for building web components:

http://www.polymer-project.org/polymer.html

We for starters, this entire component package is lighter than some single jQuery UI plugins. It is declarative, something jQuery UI simply cannot do as it is designed. Web Components APIs are landing in browsers, so these code paths will be highly optimized as time goes on - jQuery's plugin code path will not be.