Merge pull request #461 from koenkivits/patch-1

Added some Known Issues to README

Author: dfreedm

README.md

@@ -90,11 +90,35 @@ window.addEventListener('WebComponentsReady', function(e) {
 
 ## Known Issues
 
+  * [Limited CSS encapsulation](#encapsulation)
+  * [Element wrapping / unwrapping limitations](#wrapping)
   * [Custom element's constructor property is unreliable](#constructor)
   * [Contenteditable elements do not trigger MutationObserver](#contentedit)
   * [ShadowCSS: :host-context(...):host(...) doesn't work](#hostcontext)
+  * [ShadowCSS: :host(.zot:not(.bar:nth-child(2))) doesn't work](#nestedparens)
+  * [HTML imports: document.currentScript doesn't work as expected](#currentscript)
   * [execCommand isn't supported under Shadow DOM](#execcommand)
 
+### Limited CSS encapsulation <a id="encapsulation"></a>
+Under native Shadow DOM, CSS selectors cannot cross the shadow boundary. This means document level styles don't apply to shadow roots, and styles defined within a shadow root don't apply outside of that shadow root. [Several selectors](http://www.html5rocks.com/en/tutorials/webcomponents/shadowdom-201/) are provided to be able to deal with the shadow boundary.
+
+The Shadow DOM polyfill can't prevent document styles from leaking into shadow roots. It can, however, encapsulate styles within shadow roots to some extent. This behavior isn't automatically emulated by the Shadow DOM polyfill, but it can be achieved by manually using the included ShadowCSS shim:
+
+```
+WebComponents.ShadowCSS.shimStyling( shadowRoot, scope );
+```
+
+... where `shadowRoot` is the shadow root of a DOM element, and `scope` is the name of the scope used to prefix the selectors. This removes all `<style>` elements from the shadow root, rewrites it rules using the given scope and reinserts the style as a document level stylesheet. Note that the `:host` and `:host-context` pseudo classes are also rewritten.
+
+For a full explanation on the implementation and both the possibilities and the limitations of ShadowCSS please view the documentation in the [ShadowCSS source](src/ShadowCSS/ShadowCSS.js).
+
+### Element wrapping / unwrapping limitations <a id="wrapping"></a>
+The Shadow DOM polyfill is implemented by [wrapping](http://webcomponents.org/polyfills/shadow-dom/#wrappers) DOM elements whenever possible. It does this by wrapping methods like `document.querySelector` to return wrapped DOM elements. This has a few caveats:
+   * Not _everything_ can be wrapped. For example, elements like `document`, `window`, `document.body`, `document.fullscreenElement` and others are non-configurable and thus cannot be overridden.
+   * Wrappers don't support [live NodeLists](https://developer.mozilla.org/en-US/docs/Web/API/NodeList#A_sometimes-live_collection) like `HTMLElement.childNodes` and `HTMLFormElement.elements`. All NodeLists are snapshotted upon read. See [#217](https://github.com/webcomponents/webcomponentsjs/issues/217) for an explanation.
+
+In order to work around these limitations the polyfill provides the `ShadowDOMPolyfill.wrap` and `ShadowDOMPolyfill.unwrap` methods to respectively wrap and unwrap DOM elements manually.
+
 ### Custom element's constructor property is unreliable <a id="constructor"></a>
 See [#215](https://github.com/webcomponents/webcomponentsjs/issues/215) for background.
 
@@ -119,6 +143,12 @@ don't work, despite working under native Shadow DOM. The solution is to use `pol
 polyfill-next-selector { content: '.foo :host.bar, :host.foo.bar'; }
 ```
 
+### ShadowCSS: :host(.zot:not(.bar:nth-child(2))) doesn't work <a id="nestedparens"></a>
+ShadowCSS `:host()` rules can only have (at most) 1-level of nested parentheses in its argument selector under ShadowCSS. For example, `:host(.zot)` and `:host(.zot:not(.bar))` both work, but `:host(.zot:not(.bar:nth-child(2)))` does not. 
+
+### HTML imports: document.currentScript doesn't work as expected <a id="currentscript"></a>
+In native HTML Imports, document.currentScript.ownerDocument references the import document itself. In the polyfill use document._currentScript.ownerDocument (note the underscore).
+
 ### execCommand and contenteditable isn't supported under Shadow DOM <a id="execcommand"></a>
 See [#212](https://github.com/webcomponents/webcomponentsjs/issues/212)