Injects CSS into a page.

Note: When using Manifest V3 or higher, use scripting.insertCSS() and scripting.removeCSS() to insert and remove CSS.

To use this API you must have the permission for the page's URL, either explicitly as a host permission, or using the activeTab permission.

You can only inject CSS into pages whose URL can be expressed using a match pattern: meaning, its scheme must be one of "http", "https", or "file". This means that you can't inject CSS into any of the browser's built-in pages, such as about:debugging, about:addons, or the page that opens when you open a new empty tab.

Note: Firefox resolves URLs in injected CSS files relative to the CSS file itself, rather than to the page it's injected into.

The inserted CSS may be removed again by calling tabs.removeCSS().

This is an asynchronous function that returns a Promise (on Firefox only).


let inserting = browser.tabs.insertCSS(
  tabId,           // optional integer
  details          // object


tabId Optional

integer. The ID of the tab in which to insert the CSS. Defaults to the active tab of the current window.


An object describing the CSS to insert. It contains the following properties:

allFrames Optional

boolean. If true, the CSS will be injected into all frames of the current page. If it is false, CSS is only injected into the top frame. Defaults to false.

code Optional

string. Code to inject, as a text string.

cssOrigin Optional

string. This can take one of two values: "user", to add the CSS as a user stylesheet or "author" to add it as an author stylesheet. If this option is omitted, the CSS is added as an author stylesheet.

  • "user" enables you to prevent websites from overriding the CSS you insert: see Cascading order.
  • "author" stylesheets behave as if they appear after all author rules specified by the web page. This behavior includes any author stylesheets added dynamically by the page's scripts, even if that addition happens after the insertCSS call completes.
file Optional

string. Path to a file containing the code to inject. In Firefox, relative URLs are resolved relative to the current page URL. In Chrome, these URLs are resolved relative to the extension's base URL. To work cross-browser, you can specify the path as an absolute URL, starting at the extension's root, like this: "/path/to/stylesheet.css".

frameId Optional

integer. The frame where the CSS should be injected. Defaults to 0 (the top-level frame).

matchAboutBlank Optional

boolean. If true, the code will be injected into embedded "about:blank" and "about:srcdoc" frames if your extension has access to their parent document. The code cannot be inserted in top-level about: frames. Defaults to false.

runAt Optional

extensionTypes.RunAt. The soonest that the code will be injected into the tab. Defaults to "document_idle".

Return value

A Promise that will be fulfilled with no arguments when all the CSS has been inserted. If any error occurs, the promise will be rejected with an error message.


This example inserts into the currently active tab CSS which is taken from a string.

let css = "body { border: 20px dotted pink; }";

browser.browserAction.onClicked.addListener(() => {
  function onError(error) {
    console.log(`Error: ${error}`);

  let insertingCSS = browser.tabs.insertCSS({ code: css });
  insertingCSS.then(null, onError);

This example inserts CSS which is loaded from a file packaged with the extension. The CSS is inserted into the tab whose ID is 2:

browser.browserAction.onClicked.addListener(() => {
  function onError(error) {
    console.log(`Error: ${error}`);

  let insertingCSS = browser.tabs.insertCSS(2, { file: "content-style.css" });
  insertingCSS.then(null, onError);

Example extensions

Browser compatibility

BCD tables only load in the browser

Note: This API is based on Chromium's chrome.tabs API. This documentation is derived from tabs.json in the Chromium code.