A "web component" is some stand-alone combination of HTML/CSS/JS that does some thing. It should not conflict with the application that uses it in any way. It should, however, allow the application to communicate with it (to modify its state) and to potentially change its appearance in a way that will not break the component.
A problem with HTML/JS/CSS pre-shadow DOM is that it was impossible to actually build a component as described above. There was no way to sandbox the HTML/JS/CSS that's used to build the component from the application. There was always the possibility for contamination.
What the specs that Polymer is intended to polyfill permit is a combination of this sandboxing capability along with other means of permeating this sandbox with the functionality needed for such sandboxed components to be truly useful (how useful is something you embed in your page then can't access/talk to at all?). This includes data binding, events, allowing parent CSS to apply to certain things (in a very explicit, named way, which allows the parent application to style the component in a manner the component creator deems appropriate, and not in a way that will break the component), etc.
These standards, in effect, define features that once implemented will permit true web components to be built.
A problem with HTML/JS/CSS pre-shadow DOM is that it was impossible to actually build a component as described above. There was no way to sandbox the HTML/JS/CSS that's used to build the component from the application. There was always the possibility for contamination.
What the specs that Polymer is intended to polyfill permit is a combination of this sandboxing capability along with other means of permeating this sandbox with the functionality needed for such sandboxed components to be truly useful (how useful is something you embed in your page then can't access/talk to at all?). This includes data binding, events, allowing parent CSS to apply to certain things (in a very explicit, named way, which allows the parent application to style the component in a manner the component creator deems appropriate, and not in a way that will break the component), etc.
These standards, in effect, define features that once implemented will permit true web components to be built.
For more: http://www.html5rocks.com/en/tutorials/webcomponents/shadowd...