# ShadyCSS ShadyCSS provides a library to simulate ShadowDOM style encapsulation (ScopingShim), a shim for the proposed CSS mixin `@apply` syntax (ApplyShim), and a library to integrate document-level stylesheets with both of the former libraries (CustomStyleInterface). ## Requirements ShadyCSS requires support for the `` element, ShadowDOM, MutationObserver, Promise, and Object.assign ## Loading ShadyCSS can be used by loading the ScopingShim, ApplyShim, CustomStyleInterface, or any combination of those. The most-supported loading order is: 1. ScopingShim 1. ApplyShim 1. CustomStyleInterface All libraries will expose an object on `window` named `ShadyCSS` with the following interface: ```js ShadyCSS = { prepareTemplate(templateElement, elementName, elementExtension){}, styleElement(element){}, styleSubtree(element, overrideProperties){}, styleDocument(overrideProperties){}, getComputedStyleValue(element, propertyName){ return // style value for property name on element }, nativeCss: Boolean, nativeShadow: Boolean } ``` ## About ScopingShim ScopingShim provides simulated ShadyDOM style encapsulation, and a shim for CSS Custom Properties. ScopingShim works by rewriting style contents and transforming selectors to enforce scoping. Additionally, if CSS Custom Properties is not detected, ScopingShim will replace CSS Custom Property usage with realized values. ### Example: Here's an example of a custom element when Scoping Shim is not needed. ```html Shadow Light ``` becomes: ```html Shadow Light ``` ## About ApplyShim ApplyShim provides a shim for the `@apply` syntax proposed at https://tabatkins.github.io/specs/css-apply-rule/, which expands the definition CSS Custom Properties to include objects that can be applied as a block. This is done by transforming the block definition into a set of CSS Custom Properties, and replacing uses of `@apply` with consumption of those custom properties. ### Status: The `@apply` proposal has been abandoned in favor of the ::part/::theme [Shadow Parts spec](https://tabatkins.github.io/specs/css-shadow-parts/). Therefore, the ApplyShim library is deprecated and provided only for backwards compatibility. Support going forward will be limited to critical bug fixes. ### Known Issues: * Mixin properties cannot be modified at runtime. * Nested mixins are not supported. * Shorthand properties are not expanded and may conflict with more explicit properties. Whenever shorthand notations are used in conjunction with their expanded forms in `@apply`, depending in the order of usage of the mixins, properties can be overridden. This means that using both `background-color: green;` and `background: red;` in two separate CSS selectors can result in `background-color: transparent` in the selector that `background: red;` is specified. ```css #nonexistent { --my-mixin: { background: red; } } ``` with an element style definition of ```css :host { display: block; background-color: green; @apply(--my-mixin); } ``` results in the background being `transparent`, as an empty `background` definition replaces the `@apply` definition. For this reason, we recommend avoiding shorthand properties. ### Example: Here we define a block called `--mixin` at the document level, and apply that block to `my-element` somewhere in the page. ```css html { --mixin: { border: 2px solid black; background-color: green; } } my-element { border: 1px dotted orange; @apply --mixin; } ``` becomes: ```css html { --mixin_-_border: 2px solid black; --mixin_-_background-color: green; } my-element { border: var(--mixin_-_border, 1px dotted orange); background-color: var(--mixin_-_background-color); } ``` ## About CustomStyleInterface CustomStyleInterface provides API to process ` This text will be brown! ``` Another example with a wrapper `` element ```html This this text will be brown! ``` Another example with a function that produces style elements ```html This this text will be brown! ``` ## Usage To use ShadyCSS: 1. First, call `ShadyCSS.prepareTemplate(template, name)` on a `` element that will be imported into a `shadowRoot`. 2. When the element instance is connected, call `ShadyCSS.styleElement(element)` 3. Create and stamp the element's shadowRoot 4. Whenever dynamic updates are required, call `ShadyCSS.styleSubtree(element)`. 5. If a styling change is made that may affect the whole document, call `ShadyCSS.styleDocument()`. The following example uses ShadyCSS and ShadyDOM to define a custom element. ```html Content ``` ## Type Extension elements ShadyCSS can also be used with type extension elements by supplying the base element name to `prepareTemplate` as a third argument. ### Example ```html Content ``` ## Imperative values for Custom properties To set the value of a CSS Custom Property imperatively, `ShadyCSS.styleSubtree` and `ShadyCSS.styleDocument` support an additional argument of an object mapping variable name to value. When using ApplyShim, defining new mixins or new values for current mixins imperatively is not supported. ### Example ```html Text Text ``` ## Limitations ### Selector scoping To use the `::slotted` pseudo-element, you must select it as a descendant of some context element. ```css /* Bad */ ::slotted() {} /* Good */ .context ::slotted() {} ``` Since ShadyCSS removes all `` elements, you cannot select them directly or use any other selectors along with the `::slotted` pseudo-element selector. ```html ``` ```html ``` ### Custom properties and `@apply` Dynamic changes are not automatically applied. If elements change such that they conditionally match selectors they did not previously, `ShadyCSS.styleDocument()` must be called. For a given element's shadowRoot, only 1 value is allowed per custom properties. Properties cannot change from parent to child as they can under native custom properties; they can only change when a shadowRoot boundary is crossed. To receive a custom property, an element must directly match a selector that defines the property in its host's stylesheet. ### `` Flash of unstyled content If `ShadyCSS.applyStyle` is never called, `` elements will process after HTML Imports have loaded, after the document loads, or after the next paint. This means that there may be a flash of unstyled content on the first load. ### Mixins do not cascade throught `` Crawling the DOM and updating styles is very expensive, and we found that trying to update mixins through `` insertion points to be too expensive to justify for both polyfilled CSS Mixins and polyfilled CSS Custom Properties. ### External stylesheets not currently supported External stylesheets loaded via `` within a shadow root or `@import` syntax inside a shadow root's stylesheet are not currently shimmed by the polyfill. This is mainly due to the fact that shimming them would require a fetch of the stylesheet text that is async cannot be easily coordinated with the upgrade timing of custom elements using them, making it impossible to avoid "flash of unstyled content" when running on polyfill. ### Document level styling is not scoped by default ShadyCSS mimics the behavior of shadow dom, but it is not able to prevent document level styling to affect elements inside a shady dom. Global styles defined in `index.html` or any styles not processed by ShadyCSS will affect all elements on the page. To scope document level styling, the style must be wrapped in the `` element found in Polymer, or use the `CustomStyleInterface` library to modify document level styles. ### Dynamically created styles are not supported ShadyCSS works by processing a template for a given custom element class. Only the style elements present in that template will be scoped for the custom element's ShadowRoot.