Injects CSS into a page.

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

You can only inject CSS into pages whose URL can be expressed using amatch 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.

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

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

Syntax

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

Parameters

tabIdOptional

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

details

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

allFramesOptional

boolean. Iftrue, the CSS will be injected into all frames of the current page. If it isfalse, CSS is only injected into the top frame. Defaults tofalse.

codeOptional

string. Code to inject, as a text string.

cssOriginOptional

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: seeCascading 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 theinsertCSS call completes.

fileOptional

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".

frameIdOptional

integer. The frame where the CSS should be injected. Defaults to0 (the top-level frame).

matchAboutBlankOptional

boolean. Iftrue, 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 tofalse.

runAtOptional

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

Return value

APromise 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.

Examples

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

• apply-css
• beastify
• session-state

Browser compatibility