The adoptedStyleSheets property of the Document interface is used for setting an array of constructed stylesheets to be used by the document.

The same constructed stylesheets can also be shared with one or more ShadowRoot instances using the ShadowRoot.adoptedStyleSheets property. Changing an adopted stylesheet will affect all the objects that adopt it.

Stylesheets in the property are evaluated along with the document's other stylesheets using the CSS cascade algorithm. Where the resolution of rules considers stylesheet order, adoptedStyleSheets are assumed to be ordered after those in Document.styleSheets.

Only stylesheets created using the CSSStyleSheet() constructor within the context of the current Document 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 same Document.

If the array needs to be modified, then a new array must be assigned (in-place mutations like push() will throw an exception). Note however that the CSSStyleSheet() instances themselves can be modified, and these changes will apply wherever the stylesheet is adopted.

NotAllowedError DOMException

One of the CSSStyleSheet instances in the array was not created using the CSSStyleSheet() constructor or was constructed in a different document than the current document, such as one in a frame.

The code below shows a stylesheet being constructed, and then CSSStyleSheet.replaceSync() is called to add a rule to the sheet. The stylesheet is then added to an array and assigned to the adoptedStyleSheets property.

// Create an empty "constructed" stylesheet
const sheet = new CSSStyleSheet();
// Apply a rule to the sheet
sheet.replaceSync("a { color: red; }");

// Apply the stylesheet to a document
document.adoptedStyleSheets = [sheet];

We can append a new rule to the stylesheet using CSSStyleSheet.insertRule().

sheet.insertRule("* { background-color: blue; }");
// The document will now have blue background.

To append a whole new stylesheet to the adoptedStyleSheets property we have to create and assign a new combined array. This is demonstrated below using spread-syntax:

const extraSheet = new CSSStyleSheet();
extraSheet.replaceSync("p { color: green; }");

// Combine the existing sheets and new one
document.adoptedStyleSheets = [...document.adoptedStyleSheets, extraSheet];

We can share a stylesheet to a shadow root in a similar way.

// Create an element in the document and then create a shadow root:
const node = document.createElement("div");
const shadow = node.attachShadow({ mode: "open" });

//Adopt the same sheet into the shadow DOM
shadow.adoptedStyleSheets = [sheet];

• Constructable Stylesheets (web.dev)
• Using the Shadow DOM
• CSSStyleSheet() constructor
• CSSStyleSheet.replaceSync()
• CSSStyleSheet.replace()
• CSSStyleSheet.insertRule()
• CSSStyleSheet.deleteRule()