Why Your Squarespace Custom CSS Stopped Working (And How to Fix It)

Squarespace periodically updates its DOM structure and class names. Custom CSS that targets specific classes often breaks after platform updates because the selectors no longer match the live markup.

The Problem

You wake up to an email from a client: “The site looks broken.” You open the live URL and your carefully crafted custom CSS is suddenly invisible. The header is misaligned, the fonts are wrong, and the mobile menu is unreadable. Nothing changed in the Custom CSS panel — but Squarespace changed underneath it.

Why Classes Change

Squarespace deploys updates to its template engine continuously. These updates can:

• Rename CSS classes (e.g., .sqs-block → .css-1a2b3c)
• Restructure HTML nesting
• Remove wrapper elements that your CSS depended on

Squarespace does not maintain a public changelog for DOM changes. Custom code is always at risk.

How to Write Future-Proof CSS

1. Prefer Structural Selectors

Instead of:

.sqs-block-code .code-block {
  margin-top: 2rem;
}

Use:

[data-block-type="code"] {
  margin-top: 2rem;
}

data-* attributes are far more stable than auto-generated class names.

2. Avoid Deep Nesting

Deeply nested selectors break when any ancestor changes. Keep your selectors flat:

/* Bad */
body > #siteWrapper > main > .sections > .page-section > .content-wrapper

/* Good */
.page-section .content-wrapper

3. Test After Every Update

Run your site through the Sitector audit tool after any Squarespace platform update. It checks for broken selectors, layout shifts, and performance regressions in under 60 seconds.