csswg-drafts/css-shadow-parts-1/Overview.bs at 82d7b520f417923224059b960e6b1ae183fc2fec · w3c/csswg-drafts · GitHub

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<pre class='metadata'>
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 <a>shadow hosts</a>, allowing <a>shadow hosts</a> to selectively expose chosen elements from their <a>shadow tree</a> to the outside page for styling purposes.
</pre>

<pre class=link-defaults>
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
</pre>

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 <em>wants</em> to do so.

This specification defines the ''::part()'' and ''::theme()'' pseudo-elements,
which allow an author to style specific, purposely exposed elements in a <a>shadow tree</a>
from the outside page's context.
In combination with <a>custom properties</a>,
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 <em>too powerful</em> 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 <a>live profile</a>.

This left us with using <a>custom properties</a> as the only way to style into a shadow tree:
the component would advertise that it uses certain <a>custom properties</a> to style its internals,
and the outer page could then set those properties as it wished on the <a>shadow host</a>,
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 <a>custom properties</a>
(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 <a>custom properties</a> 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 <a>custom property</a> 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 <a>custom properties</a>,
however,
is that inheritance doesn't stop at the first shadow tree.
Unless explicitly blocked,
a <a>custom property</a> 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 <em>absolutely zero new theoretical power</em>.
They are not a rehash of the ''>>>'' combinator,
they're simply a more convenient and consistent syntax
for something authors can already do with <a>custom properties</a>.
By separating out the explicitly "published" parts of an element
(the <a>shadow part map</a>
from the sub-parts that it merely happens to contain
(the <a>computed shadow theme map</a>,
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 <dfn element-attr for=html-global>part</dfn> attribute.
This is used to expose the element outside the <a>shadow tree</a>,
and to "forward" sub-parts of the element
(if it has its own <a>shadow tree</a>)
to outside the <a>shadow tree</a>.

The part attribute is parsed as a comma-separated list of part mappings.
Each part mapping is one of:

<dl class=switch>
    : <code>ident</code>
    :: Adds «[ ident → el ]» to the <a>shadow part map</a> of the <a>shadow root</a> containing el.

    : <code>ident1 => ident2</code>
    :: If el is a <a>shadow host</a>,
        and it's <a>shadow root's</a> <a>shadow part map</a> |partMap| [=map/contains=] ident1,
        then this adds «[ ident2 → |partMap|[ident1] ]» to the <a>shadow part map</a> of the <a>shadow root</a> containing el.

    : <code>* => prefix*</code>
    :: If el is a <a>shadow host</a>,
        then [=map/for each=] |ident| → |subEl| in el's <a>shadow root's</a> <a>shadow part map</a>,
        «[ prefix + |ident| → |subEl| ]» is added to the <a>shadow part map</a> of the <a>shadow root</a> containing el.

    : anything else
    :: Ignored for error-recovery / future compat.
</dl>

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 <a>shadow root</a> has a <dfn export for="shadow root">shadow part map</dfn>
which is an <a>ordered map</a>,
with keys that are [=strings=]
and values that are [=ordered sets=] of elements.

The <a>shadow part map</a> contains all the entries described by the elements in its <a>shadow tree</a>,
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 <dfn export for="shadow root">partial shadow theme map</dfn>
and a <dfn export for="shadow root">computed shadow theme map</dfn>
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 <dfn element-attr for=html-global>theme</dfn> 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 <dfn selector>::part()</dfn> and <dfn selector>::theme()</dfn> pseudo-elements
(collectively, the <dfn export>shadow-part pseudo-elements</dfn>)
allow you to select elements that have been exposed via a <{html-global/part}> attribute.
The syntaxes of them are:

<pre class=prod>
    ::part() = ::part( <<ident>> )
    ::theme() = ::theme( <<ident>> )
</pre>

The ''::part()'' pseudo-element only matches anything
when the <a>originating element</a> is a <a>shadow host</a>.
If the <a>originating element's</a> <a>shadow root's</a> <a>shadow part map</a>
[=map/contains=] the specified <<ident>>,
''::part()'' matches the element or elements keyed to that <<ident>>.
Otherwise, it matches nothing.

<div class="example">
    For example,
    if you have a custom button
    that contains a "label" element that is exposed for styling
    (via <code>part="label"</code>),
    you can select it with
    ''#the-button::part(label)''.
</div>

The ''::theme()'' pseudo-element is similar,
except it can match regardless of whether the <a>originating element</a>
is a <a>shadow host</a> or not.
It matches the elements keyed to the specified <<ident>>
in the <a>computed shadow theme map</a> of the <a>shadow trees</a>
of the <a>originating element</a> or any descendants.

<div class="example">
    For example,
    '':root::theme(label)'' matches any element with <code>part="label"</code>
    anywhere in the entire document,
    no  matter how deeply nested into shadow trees they are.
</div>

The <a>shadow-part pseudo-elements</a> can take additional pseudo-classes after them,
such as ''x-button::part(label):hover'',
but never match the <a>structural pseudo-classes</a>
or any other pseudo-classes that match based on tree information
rather than local element information.

The <a>shadow-part pseudo-elements</a> also can take additional pseudo-elements after them,
such as ''x-button::part(label)::before'',
but never match additional <a>shadow-part pseudo-elements</a>.

<div class=example>
    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 <code>&lt;x-panel></code>'s internal confirm button had used something like
    <code>part="confirm-button, * => confirm-*"</code>
    to forward the button's internal parts up into the panel's own <a>shadow part map</a>,
    then a selector like
    ''x-panel::part(confirm-label)''
    would select just the one button's label,
    ignoring any other labels.
</div>