# Custom CSS & JavaScript Drop bespoke CSS and JS into a test or variant when the visual editor isn't enough. Variant-scoped runs only for that variant; test-wide runs for everyone in the test (including the control). A power-user escape hatch for changes the visual editor can't express. Drop bespoke CSS or JavaScript onto a variant — or onto every visitor in the test — and the SDK injects it on the relevant page loads. Custom CSS and JavaScript live on the same screen as your variants in the test wizard, but most teams don't need them — the visual editor handles 90% of variant authoring without custom code. Reach for these fields when the change is dynamic, computed, or easier to express as a stylesheet rule than a list of DOM mutations. The SDK applies custom CSS and JS automatically as part of the same DOM-mutation step that powers visual variants. There's nothing to wire up on your side — paste the code, save the variant, and the SDK takes care of injection and cleanup. ## Two scopes ### Variant-level Set on a single variant. Runs only for visitors assigned to that variant. This is where the actual difference between variants usually lives — most teams use variant-level custom code, not test-level. ### Test-level Set on the test itself. Runs for every visitor in the test, including the control. Useful when every variant should get the same supporting code — a CSS reset, an instrumentation snippet, a helper function. Rare in practice. ## How injection works - **CSS** — injected as a `