Let's take a look at the original list of use cases proposed in 2008 at http://www.w3.org/2008/webapps/wiki/Component_Model_Use_Cases:
Layout Manager Layout Manager Use Case Parameters Who Web Framework Engineer What Build a layout library, consisting of a UI layout primitives, such as panel, resizeable panel, tab group, stack, accordion containers, etc. Purpose Allow web application developers to easily build constrained layout and populate it with arbitrary HTML content. Provide a way for developers to build their own layout primitives by extending the provided ones. Examples Dijit Layout GWT Panels Sencha Panels SproutCore Views Desirable Properties Composability -- a method to compose layouts with both UI primitives and DOM elements. Extensibility -- a way to build on layout primitives to create new ones. Encapsulation -- styles, defined to hold layout in place should not be in danger of being stomped on by the author stylesheets. Performance -- layouts built with layout manager should appear quickly, avoiding re-flows and FOUC-like symptoms. Widget Mix-and-Match This use case appears to be satisfied by the existing specifications but only if we use custom elements and shadow DOM together. Mix-and-matching Use Case Parameters Who Web Application Engineer What Build an application using multiple existing controls from several frameworks. Purpose Write application faster by reusing code. Avoid having to exclusively go with one framework. Examples How do you access Dijit elements in JQuery Using SproutCore 2.0 with jQuery UI Which JavaScript framework (jQuery vs Dojo vs … )? Desirable Properties Consistency -- a compatible way to expose properties and methods across widget frameworks. Composability -- a method to compose with cross-framework widgets. Rendering Form Controls with SVG This use case is not satisfied by existing specifications because consistency and composability requirements are not met. SVG Form Controls Use Case Parameters Who Web Application Engineer or Web Framework Engineer What Create a set of UI controls that act like standard HTML forms controls, but use SVG for rendering. Purpose Provide custom appearance of form controls, consistent with overall the Web application theme. Make the form controls scale beautifully for various resolutions and form factors. Examples SproutCore's ImageButtonView, Sencha's Number -- examples of extensive themed form control hierarchy. Desirable Properties Consistency -- the controls should act just like any other DOM elements. Encapsulation -- the document shouldn't be able to accidentally mess up the rendering of the controls. Performance -- load quickly, avoid FOUC-like symptoms when using controls. This use case is probably satisfied by using shadow DOM. Contacts Widge Contacts Widget Use Case Parameters Who Web Application Engineer What Build a drop-in Contacts widget, which has a pre-defined appearance and shows a list of your contacts, with a way to change the widget to compact or full view and to tell the widget to refresh its state. Purpose Use the widget anywhere in the application without having to worry about styles affecting its appearance. Hide details of loading contact data and other plumbing of the widget from the consuming code with a stable API. Examples A screenshot of Google+ "in your circles" widget Desirable Properties Encapsulation -- means to ensure style of the document does not affect the widget, and widget's logic is kept to the widget. Composability -- easily added anywhere in the DOM tree. Again, this use case is satisfied only if we used both custom elements and shadow DOM together. Like/+1 Button Like/+1 Button Use Case Parameters Who Web Application Engineer What Build a drop-in widget with a pre-defined appearance of a button, showing a count of likes/+1s for this instance of a button (count is stored at a central location), embeddable on any document on the Web. Purpose Provide a simple vehicle for Web authors to embed the button. Isolate widget implementation details from the document. Examples Facebook Like button embedding instructions Google +1 button embedding instructions Desirable Properties Encapsulation -- means to ensure style of the document does not affect the widget, and widget's logic is kept to the widget. Confinement -- a way to completely isolate the widget implementation from the document in which it is being embedded. Performance -- don't block the page load. This use case is not satisfied due to confinement requirement. Table-based Charts Table-based Charts Use Case Parameters Who Web Framework Engineer What Provide a way to represent table data markup as charts or diagrams. Purpose Make it easy for Web authors to create charts and diagrams using table markup. Examples Christian Heilmann's tochart script Desirable Properties Composability -- one should be able to make chart by creating a table, imperatively or declaratively. Performance -- no FOUC or blocking load when charts are loaded. This use case is half-satisified since decorator hasn't been spec'ed yet. You can combine custom elements and shadow DOM but that's hardly easy. Timezone selection via Image Timezone selection via Image Use Case Parameters Who Web Framework Engineer What Graphical representation of a timezone selector that shows a world map in addition to/instead of a drop-down list. Purpose Make it easy for Web authors to spruce up time zone selection (or similar). Examples Time Zone Map Desirable Properties Extensibility -- Basically extending <select>. Should fall back to a simple <select> where components are not supported. Consistency -- Extend the <select> API for item selection. Again, this use case is half satisfied in that is=~ attribute hasn't been spec'ed although you can imperatively attach shadow DOM. Entry-helper Entry-helper Use Case Parameters Who Web Framework Engineer, Web Application Engineer What Add an entry-helper (drop-down) list to input fields. Purpose Help the user fill in a form field, show suggestions and acceptable values, speed up data entry. Examples Most browser's address bar or web search field Desirable Properties Extensibility -- Basically extending <input>. Should fall back to a simple <input> where components are not supported. Ditto. I don't think "Built-in HTML Elements Use Case Parameters" is a valid use case as it has been discussed numerous times. So again, I don't see any use case listed there that could be met by simply introducing document.register. - R. Niwa
