Boosts

Every site has one thing you'd change.
Change it once, and keep it.

Per-site CSS and JavaScript for any Chromium browser.
No account, no cloud, no build step, no dependencies.

boosts.krawo.li

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A docs page with a column too narrow to read. A sidebar you have never once looked at. A header that follows you down the page, eating a third of the screen. A font two sizes too small on the one dashboard you stare at all day.

You notice them every single day, and you fix them never. Fixing them means opening DevTools, and DevTools forgets the moment you reload.

Boosts remembers. Write a snippet of CSS or JavaScript, point it at the sites it belongs to, and it is applied every time you visit them. Then stop thinking about it.

What it does

A boost is a snippet of CSS, JavaScript, or both, plus a list of the sites it runs on. That is the whole model. Everything below is in service of making that quick to write and hard to get wrong.

• CSS and JavaScript, per boost, with a real editor: line numbers, auto-indent, bracket completion and syntax highlighting for both languages.
• CSS applies live. Save and every open tab updates, with no reload. The popup previews it as you type.
• Friendly site patterns. Type github.com, get *://*.github.com/*. Type localhost:3000 and the port is dropped for you, because match patterns already cover every port.
• Everything autosaves, because a popup is dismissed by clicking away and there is no chance to press a button.
• Nothing leaves the browser. No network calls of any kind: not for sync, not for telemetry, not for updates.

Install

Boosts is not on the Chrome Web Store. Load it from source:

1. Clone the repository, or download it as a ZIP and unpack it.

   git clone https://github.com/o-ba/boosts.git

2. Open chrome://extensions.

3. Turn on Developer mode, top right.

4. Click Load unpacked and pick the folder you just cloned.

The manager opens by itself the first time.

Works in Chrome, Edge, Brave, Vivaldi, Arc and anything else Chromium-based, version 120 or newer.

Enabling JavaScript

CSS works immediately. Chromium requires a separate opt-in before any extension may run user JavaScript, and it is off by default even for unpacked extensions:

1. On chrome://extensions, open Details for Boosts.
2. Switch on Allow User Scripts.
3. Back in the manager, press Re-check.

The manager shows a banner with these steps until the toggle is on.

Using it

Popup

Click the toolbar icon, or press Alt+Shift+B. It edits the boost that applies to the site you are looking at, previews CSS live as you type, and offers a new boost pre-filled with the current host if nothing matches yet.

Manager

Manage in the popup, or the extension's options page. Everything lives here: all boosts, site patterns, run timing, JS context, ordering, import and export.

Pause all

Top right of the manager. Stops every boost without disabling them one at a time; the toolbar badge shows a pause mark while paused.

This is the way out if a boost with a wide pattern breaks the pages you need. The manager is an extension page, so no boost can reach it. It stays usable even when every site is broken.

Site patterns

One per line. Friendly input is expanded into a Chrome match pattern, and the manager shows the result underneath the field as you type.

You type	It becomes	Matches
github.com	*://*.github.com/*	github.com and all subdomains
gist.github.com	*://*.gist.github.com/*	that host and its subdomains
example.com/docs*	*://*.example.com/docs*	paths starting with /docs
*.example.com	*://*.example.com/*	unchanged, already a wildcard host
localhost:3000	*://*.localhost/*	localhost on any port
https://news.ycombinator.com/*	unchanged	full patterns pass through
*	<all_urls>	everything

A bare host always includes its subdomains, which is what people usually mean.

Chrome match patterns have no port syntax, and a pattern without a port already matches every port, so a typed port is dropped and the manager says so. Anything that cannot be rescued is refused with a reason you can act on, site names cannot contain spaces, "about:" pages cannot be modified by any extension, rather than being stored and failing silently later. An enabled boost that ends up with no usable pattern is flagged in the list.

Options per boost

Option	Choices	Notes
Run JS at	document_start, document_end, document_idle	CSS is always applied at document_start, so there is no flash of unstyled page
JS context	Page, Isolated	Page reaches page globals such as window.app. Isolated shares only the DOM
Frames	off by default	So boosts do not run inside every ad iframe

Boosts are an ordered list, and that order decides which rule wins when two of them style the same element. The arrows on each row change it.

Import and export

Export writes every boost to a single JSON file. Import reads one back.

Import is the one path that ingests a file someone else wrote, so it is treated as such: everything arrives switched off, and the manager shows what the file is asking for: How many boosts, which sites, and whether any of them run JavaScript, before anything is written to storage.

How it works

• The service worker turns each stored boost into chrome.userScripts registrations: one for the CSS at document_start, one for the JS at its own timing. They go in as separate register() calls, so a pattern Chrome rejects on one cannot take the other down, and one bad boost cannot take the rest with it.
• A resync registers the new set before removing stale ids, so a page loading while the service worker restarts is never left unstyled.
• CSS is injected as a <style> element that a MutationObserver re-attaches if the page removes it, which is what usually breaks naive userstyle injection on single-page apps.
• Your JavaScript is wrapped in an async function, so top-level await works the way it does in the DevTools console, and given a //# sourceURL so the console names it.
• Storage writes are serialised. Autosave fires them in bursts and each one is a read-modify-write of the whole list, so two overlapping saves would otherwise lose one.
• Saves are guarded by a revision counter, so an edit that lands mid-write stays queued instead of being marked as saved.
• Everything is in chrome.storage.local. storage.sync has an 8 KB per-item quota that CSS blobs blow straight through, so the export file is the way to move boosts between machines.

The editor

A textarea with a line-number gutter, two-space indentation, auto-indent, bracket completion and syntax highlighting for CSS and JavaScript, in light and dark.

Two things worth knowing:

• Tab indents, so it cannot also move focus. Press Esc first and the next Tab leaves the editor; typing anything puts Tab back to indenting.
• No error checking. Nothing here parses your code, it only colours it. A syntax error shows up in the page console at runtime, prefixed with [Boosts].

Development

sh tests/run.sh

194 assertions across five suites, run against macOS's built-in JavaScriptCore, so there is nothing to install:

Suite	Covers
parse	every source file parses
patterns	the normaliser rescues what it can and refuses the rest, checked against the inputs Chrome actually rejects
highlight	the tokenisers never alter the text they are given, and get token types right inside at-rule blocks
store	concurrent writes do not clobber each other; imported boosts can never arrive enabled
registrations	the registration loop, against a fake chrome.userScripts that validates match patterns the way Chrome does, including that a resync never drops below full coverage

On Linux or Windows, swap the JSC path in tests/run.sh for any JavaScript shell with load(), readFile() and print().

Limitations

• chrome://, about: and Chrome Web Store pages are off limits to every extension, so boosts cannot run there.
• file:// boosts also need Allow access to file URLs in the extension's details.
• JavaScript injection depends on chrome.userScripts, which needs Chromium 120 or newer.
• No sync between machines. Use export and import.

Security

Boosts requests <all_urls> because a per-site injector cannot know in advance which sites you will want. Chrome summarises that permission as "Read your browsing history" on the details page.

What the extension does with it is all in this repository: about 3,000 lines of dependency-free JavaScript, unminified, with no network access of any kind.

Boosts runs code you wrote, or code you imported. A boost set to the Page context can read and change anything on the sites it matches. Treat an imported file the way you would treat a script you were about to paste into the console, which is why import lands everything disabled and shows you what it wants first.

Contributing

Contributions are welcome and encouraged.

License

GNU General Public License v2.0.

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Created with ❤️ by Krawoli in 2026