How can I validate that a backup of my customized Bulma Sass variables restores correctly without breaking the layout?

I want to create a reliable backup of my customized Bulma Sass variables so that I can later restore the exact styling without introducing visual regressions. Since Bulma is a CSS‑only framework that outputs a single bulma.css file and relies on autoprefixer for cross‑browser Flexbox support, the validation must focus on the generated CSS and its rendering in supported browsers.

I need an automated way to confirm that the restored bulma.css produces the same layout as the original backup across Chrome, Edge, Firefox, Opera, Safari and IE10+, preferably without executing any JavaScript.

What automated visual regression approach works best for Bulma CSS‑only projects? How can I compare the generated bulma.css before and after restore to detect unintended changes? Is there a recommended way to snapshot DOM rendering using headless browsers for validation?