Constructable Stylesheets: seamless reusable styles
Constructable Stylesheets are a new way to create and distribute reusable styles when using Shadow DOM.
It has always been possible to create stylesheets using JavaScript. However, the
process has historically been to create a <style> element using
document.createElement('style'), and then access its sheet property to obtain
a reference to the underlying
CSSStyleSheet
instance. This method can produce duplicate CSS code and its attendant bloat,
and the act of attaching leads to a flash of unstyled content whether there is
bloat or not. The CSSStyleSheet interface is the root of a collection of CSS
representation interfaces referred to as the
CSSOM,
offering a programmatic way to manipulate stylesheets as well as eliminating the
problems associated with the old method.
Constructable Stylesheets make it possible to define and prepare shared CSS styles, and then apply those styles to multiple Shadow Roots or the Document easily and without duplication. Updates to a shared CSSStyleSheet are applied to all roots into which it has been adopted, and adopting a stylesheet is fast and synchronous once the sheet has been loaded.
The association set up by Constructable Stylesheets lends itself well to a number of different applications. It can be used to provide a centralized theme used by many components: the theme can be a CSSStyleSheet instance passed to components, with updates to the theme propagating out to components automatically. It can be used to distribute CSS Custom Property values to specific DOM subtrees without relying on the cascade. It can even be used as a direct interface to the browser’s CSS parser, making it easy to preload stylesheets without injecting them into the DOM.
Constructing a StyleSheet
Rather than introducing a new API to accomplish this, the Constructable
StyleSheets specification makes
it possible to create stylesheets imperatively by invoking the CSSStyleSheet()
constructor. The resulting CSSStyleSheet object has two new methods that make it
safer to add and update stylesheet rules without triggering Flash of Unstyled
Content (FOUC).
replace()
returns a Promise that resolves once any external references (@imports) are
loaded, whereas
replaceSync()
doesn’t allow external references at all:
const sheet = new CSSStyleSheet();
// replace all styles synchronously:
sheet.replaceSync('a { color: red; }');
// this throws an exception:
try {
sheet.replaceSync('@import url("styles.css")');
} catch (err) {
console.error(err); // imports are not allowed
}
// replace all styles, allowing external resources:
sheet.replace('@import url("styles.css")')
.then(sheet => {
console.log('Styles loaded successfully');
})
.catch(err => {
console.error('Failed to load:', err);
});
Using Constructed StyleSheets
The second new feature introduced by Constructable StyleSheets is an adoptedStyleSheets property available on Shadow Roots and Documents. This lets us explicitly apply the styles defined by a CSSStyleSheet to a given DOM subtree. To do so, we set the property to an array of one or more stylesheets to apply to that element.
// Create our shared stylesheet:
const sheet = new CSSStyleSheet();
sheet.replaceSync('a { color: red; }');
// Apply the stylesheet to a document:
document.adoptedStyleSheets = [sheet];
// Apply the stylesheet to a Shadow Root:
const node =
document.createElement('div');
const shadow = node.attachShadow({ mode: 'open' });
shadow.adoptedStyleSheets = [sheet];
Notice that we’re overriding the value of adoptedStyleSheets instead of
changing the array in place. This is required because the array is frozen;
in-place mutations like push() throw an exception, so we have to assign a new
array. To preserve any existing StyleSheets added via adoptedStyleSheets, we
can use concat to create a new array that includes the existing sheets as well
as additional ones to add:
const sheet = new CSSStyleSheet();
sheet.replaceSync('a { color: red; }');
// Combine existing sheets with our new one:
document.adoptedStyleSheets = [...document.adoptedStyleSheets, sheet];
Putting it All Together
With Constructable StyleSheets, web developers now have an explicit solution for creating CSS StyleSheets and applying them to DOM trees. We have a new Promise-based API for loading StyleSheets from a string of CSS source that uses the browser’s built-in parser and loading semantics. Finally, we have a mechanism for applying stylesheet updates to all usages of a StyleSheet, simplifying things like theme changes and color preferences.
Looking Ahead
The initial version of Constructable Stylesheets is shipping with the API described here, but there’s work underway to make things easier to use. There’s a proposal to extend the adoptedStyleSheets FrozenArray with dedicated methods for inserting and removing stylesheets, which would obviate the need for array cloning and avoid potential duplicate stylesheet references.