Title: CSS Shadow Parts
Shortname: css-shadow-parts
Level: 1
Group: CSSWG
Status: ED
Work Status: exploring
URL: http://drafts.csswg.org/css-shadow-parts/
Editor: Tab Atkins-Bittner, Google, http://xanthir.com/contact/, w3cid 42199
Abstract: This specification defines the ''::part()'' and ''::theme()'' pseudo-elements on shadow hosts, allowing shadow hosts to selectively expose chosen elements from their shadow tree to the outside page for styling purposes.

spec:selectors-4;
    type:selector; text::hover
    type:dfn; text:live profile
    type:dfn; text:structural pseudo-class
spec:dom; type:dfn; for:/; text:shadow root
spec:infra;
    type:dfn; text:string
Introduction {#intro} ===================== Issue: This spec is intentionally a rough sketch at the moment. It should contain all the details necessary to evaluate the proposal, but is intentionally avoiding precise algorithms at the moment, to aid in easy comprehension and to, hopefully, discourage implementation from this sketch. Shadow DOM allows authors to separate their page into "components", subtrees of markup whose details are only relevant to the component itself, not the outside page. This reduces the chance of a style meant for one part of the page accidentally over-applying and making a different part of the page look wrong. However, this styling barrier also makes it harder for a page to interact with its components when it actually wants to do so. This specification defines the ''::part()'' and ''::theme()'' pseudo-elements, which allow an author to style specific, purposely exposed elements in a shadow tree from the outside page's context. In combination with custom properties, which let the outside page pass particular values (such as theme colors) into the component for it to do with as it will, these pseudo-elements allow components and the outside page to interact in safe, powerful ways, maintaining encapsulation without surrending all control. Motivation {#motivation} ------------------------ For obvious reasons, it's valuable to let the outside page style the internals of a shadow tree, at least in some limited ways. (The ubiquity of UA-specific pseudo-elements for the various input elements shows this.) The previous proposed method for doing so, the >>> combinator, turned out to be too powerful for its own good; it exposed too much of a component's internal structure to scrutiny, defeating some of the encapsulation benefits that using Shadow DOM brings. For this, and other performance-related reasons, the >>> combinator was eventually removed from the live profile. This left us with using custom properties as the only way to style into a shadow tree: the component would advertise that it uses certain custom properties to style its internals, and the outer page could then set those properties as it wished on the shadow host, letting inheritance push the values down to where they were needed. This works very well for many simple theming use-cases. However, there are some cases where this falls down. If a component wishes to allow arbitrary styling of something in its shadow tree, the only way to do so is to define hundreds of custom properties (one per CSS property they wish to allow control of), which is obviously ridiculous for both usability and performance reasons. The situation is compounded if authors wish to style the component differently based on pseudo-classes like '':hover''; the component needs to duplicate the custom properties used for each pseudo-class (and each combination, like '':hover:focus'', resulting in a combinatorial explosion). This makes the usability and performance problems even worse. We introduce ''::part()'' to handle this case much more elegantly and performantly. Rather than bundling everything into custom property names, the functionality lives in selectors and style rule syntax, like it's meant to. This is far more usable for both component authors and component users, should have much better performance, and allows for better encapsulation/API surface. Another interesting facet of using custom properties, however, is that inheritance doesn't stop at the first shadow tree. Unless explicitly blocked, a custom property inherits down thru nested trees, allowing authors to style deeply nested components as easily as they style directly-visible ones. The same considerations apply to this case, so we introduce ''::theme()'' to handle this. It's important to note that ''::part()'' and ''::theme()'' offer absolutely zero new theoretical power. They are not a rehash of the ''>>>'' combinator, they're simply a more convenient and consistent syntax for something authors can already do with custom properties. By separating out the explicitly "published" parts of an element (the shadow part map from the sub-parts that it merely happens to contain (the computed shadow theme map, it also helps with encapsulation, as authors can use ''::part()'' without fear of accidental over-styling. Exposing a Shadow Element: the <{html-global/part}> attribute {#part-attr} ============================================================= Any element in a shadow tree can have a part attribute. This is used to expose the element outside the shadow tree, and to "forward" sub-parts of the element (if it has its own shadow tree) to outside the shadow tree. The part attribute is parsed as a comma-separated list of part mappings. Each part mapping is one of:
: ident :: Adds «[ ident → el ]» to the shadow root's shadow part map. : ident1 => ident2 :: If el is a shadow host, and it's shadow root's shadow part map |partMap| [=map/contains=] ident1, then this adds «[ ident2 → |partMap|[ident1] ]» to the shadow root's shadow part map. : * => prefix* :: If el is a shadow host, then [=map/for each=] |ident| → |subEl| in el's shadow root's shadow part map, «[ prefix + |ident| → |subEl| ]» is added to the shadow root's shadow part map. : anything else :: Ignored for error-recovery / future compat.
Issue: Factor this out into an algorithm. Issue: The value in the map is a set of elements, not individual elements; need to properly append. Issue: Want a way to forward a name with the same name, without repeating the name (currently requires `foo => foo`). Issue: When doing prefixed-wildcard forwarding, should probably automatically exclude sub-parts that are manually forwarded. With that, would be good to have a syntax to block forwarding of a sub-part (currently would require `foo => nonsense-name`). Note: It's okay to give a part multiple names, or map a sub-part to several names. The "part name" should be considered similar to a class, not an id or tagname. Each shadow root has a shadow part map which is an ordered map, with keys that are [=strings=] and values that are [=ordered sets=] of elements. The shadow part map contains all the entries described by the elements in its shadow tree, as described above. Exposing More Widely: the <{html-global/theme}> attribute {#theme-attr} ----------------------------------------------------------------------- In addition to the [=shadow part map=], every [=shadow root=] has a partial shadow theme map and a computed shadow theme map both of which are [=ordered maps=] (with the same key/value shape as the [=shadow part map=]), and the elements in the [=shadow tree=] have a corresponding theme attribute. The <{html-global/theme}> attribute is parsed and interpreted identically to the <{html-global/part}> attribute, except that it adds its entries to the [=shadow root’s=] [=partial shadow theme map=] instead. If the [=shadow root’s=] {{ShadowRoot/mode}} is {{ShadowRootMode/"closed"}}, the [=computed shadow theme map=] is identical to the [=partial shadow theme map=]. Otherwise, it's the union of the [=partial shadow theme map=] with the [=computed shadow theme maps=] of every [=shadow host’s=] [=shadow root=] in its [=shadow tree=]. Selecting a Shadow Element: the ''::part()'' and ''::theme()'' pseudo-elements {#part-theme} ============================================================================================ The ::part() and ::theme() pseudo-elements (collectively, the shadow-part pseudo-elements) allow you to select elements that have been exposed via a <{html-global/part}> attribute. The syntaxes of them are: 
    ::part() = ::part( <> )
    ::theme() = ::theme( <> )
The ''::part()'' pseudo-element only matches anything when the originating element is a shadow host. If the originating element's shadow root's shadow part map [=map/contains=] the specified <>, ''::part()'' matches the element or elements keyed to that <>. Otherwise, it matches nothing.
For example, if you have a custom button that contains a "label" element that is exposed for styling (via part="label"), you can select it with ''#the-button::part(label)''.
The ''::theme()'' pseudo-element is similar, except it can match regardless of whether the originating element is a shadow host or not. It matches the elements keyed to the specified <> in the computed shadow theme map of the shadow trees of the originating element or any descendants.
For example, '':root::theme(label)'' matches any element with part="label" anywhere in the entire document, no matter how deeply nested into shadow trees they are.
The shadow-part pseudo-elements can take additional pseudo-classes after them, such as ''x-button::part(label):hover'', but never match the structural pseudo-classes or any other pseudo-classes that match based on tree information rather than local element information. The shadow-part pseudo-elements also can take additional pseudo-elements after them, such as ''x-button::part(label)::before'', but never match additional shadow-part pseudo-elements.
For example, ''x-panel::part(confirm-button)::part(label)'' never matches anything. This is because doing so would expose more structural information than is intended. One can still target the nested label with a selector like ''x-panel::theme(label)''. However, this will also select the labels of any other buttons in the panel. If the <x-panel>'s internal confirm button had used something like part="confirm-button, * => confirm-*" to forward the button's internal parts up into the panel's own shadow part map, then a selector like ''x-panel::part(confirm-label)'' would select just the one button's label, ignoring any other labels.