Injects CSS into a page.

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", "file", "ftp". 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.

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).


var 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:
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.
string. Code to inject, as a text string.
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.
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".
integer. The frame where the CSS should be injected. Defaults to 0 (the top-level frame).
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.
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.

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

browser.browserAction.onClicked.addListener(() => {

  function onError(error) {
    console.log(`Error: ${error}`);

  var 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}`);

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

Example extensions

Browser compatibility

Update compatibility data on GitHub
ChromeEdgeFirefoxOperaFirefox for Android
insertCSSChrome Full support YesEdge Full support 14Firefox Full support 47Opera Full support YesFirefox Android Full support 54
cssOriginChrome Full support 39Edge Full support 79Firefox Full support 53Opera No support NoFirefox Android Full support 54
frameIdChrome Full support 39Edge Full support 14Firefox Full support 47Opera Full support 26Firefox Android Full support 54
matchAboutBlankChrome Full support 39Edge Full support 14Firefox No support NoOpera Full support 26Firefox Android No support No
runAtChrome Full support 20Edge Full support 79Firefox Full support 47Opera Full support 15Firefox Android Full support 54


Full support  
Full support
No support  
No support

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

Microsoft Edge compatibility data is supplied by Microsoft Corporation and is included here under the Creative Commons Attribution 3.0 United States License.