The adoptedStyleSheets property of the ShadowRoot interface sets an array of constructed stylesheets to be used by the shadow DOM subtree.
Note: A constructed stylesheet is a stylesheet created programmatically using the CSSStyleSheet() constructor (as compared to one created by a user-agent when importing a stylesheet from a script, imported using <style> and @import, or linked to via <link>).
The same constructed stylesheet can be adopted by multiple ShadowRoot instances, and by the parent document (using the Document.adoptedStyleSheets property). Changing an adopted stylesheet will affect all the adopting objects.
Stylesheets in the adoptedStyleSheets property are considered along with the shadow DOM's other stylesheets. For the purpose of determining the final computed CSS of any element, they are considered to have been added after the other stylesheets in the shadow DOM (ShadowRoot.styleSheets).
Only stylesheets created using the CSSStyleSheet() constructor, and from within the same parent Document as the shadow root, may be adopted.
The value is an array of CSSStyleSheet() instances that must have been created using the CSSStyleSheet() constructor within the context of the shadow root's parent Document.
If the array needs to be modified, use in-place mutations like push(). Note, the CSSStyleSheet() instances themselves can also be modified, and these changes will apply wherever the stylesheet is adopted.
The code below first shows a stylesheet being constructed, and then CSSStyleSheet.replaceSync() is called to add a rule to the sheet.
const sheet = new CSSStyleSheet();
sheet.replaceSync("a { color: red; }");
We then create a ShadowRoot and pass the sheet object to adoptedStyleSheets inside an array.
const node = document.createElement("div");
const shadow = node.attachShadow({ mode: "open" });
shadow.adoptedStyleSheets = [sheet];
We can still modify the stylesheets after they have been added to the array. Below we append a new rule to the same sheet using CSSStyleSheet.insertRule().
sheet.insertRule("* { background-color: blue; }");
New stylesheets can be appended to the document or shadow root by using adoptedStyleSheets.push():
const extraSheet = new CSSStyleSheet();
extraSheet.replaceSync("p { color: green; }");
shadow.adoptedStyleSheets.push(extraSheet);